term/serial: Add support for PCI serial devices

Loosely based on early_pci_serial_init() from Linux, allow GRUB to make
use of PCI serial devices.

Specifically, my Alderlake NUC exposes the Intel AMT SoL UART as a PCI
enumerated device but doesn't include it in the EFI tables.

Tested and confirmed working on a "Lenovo P360 Tiny" with Intel AMT
enabled. This specific machine has (from lspci -vv):

00:16.3 Serial controller: Intel Corporation Device 7aeb (rev 11) (prog-if 02 [16550])
        DeviceName: Onboard - Other
        Subsystem: Lenovo Device 330e
        Control: I/O+ Mem+ BusMaster- SpecCycle- MemWINV- VGASnoop- ParErr- Stepping- SERR- FastB2B- DisINTx-
        Status: Cap+ 66MHz+ UDF- FastB2B+ ParErr- DEVSEL=fast >TAbort- <TAbort- <MAbort- >SERR- <PERR- INTx-
        Interrupt: pin D routed to IRQ 19
        Region 0: I/O ports at 40a0 [size=8]
        Region 1: Memory at b4224000 (32-bit, non-prefetchable) [size=4K]
        Capabilities: [40] MSI: Enable- Count=1/1 Maskable- 64bit+
                Address: 0000000000000000  Data: 0000
        Capabilities: [50] Power Management version 3
                Flags: PMEClk- DSI+ D1- D2- AuxCurrent=0mA PME(D0-,D1-,D2-,D3hot-,D3cold-)
                Status: D0 NoSoftRst+ PME-Enable- DSel=0 DScale=0 PME-
        Kernel driver in use: serial

From which the following config (/etc/default/grub) gets a working
serial setup:

GRUB_CMDLINE_LINUX="console=tty0 earlyprintk=pciserial,00:16.3,115200 console=ttyS0,115200"
GRUB_SERIAL_COMMAND="serial --port=0x40a0 --speed=115200"
GRUB_TERMINAL="serial console"

Documentation is added to note that serial devices found on the PCI bus will
be exposed as "pci,XX:XX.X" and how to find serial terminal logical names.
Also, some minor documentation improvements were added.

This can be tested in QEMU by adding a pci-serial device, e.g. using the option
"-device pci-serial".

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Glenn Washburn <development@efficientek.com>
Signed-off-by: Glenn Washburn <development@efficientek.com>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>

docs/grub.texi

@@ -2716,8 +2716,7 @@ grub> @kbd{terminal_input serial; terminal_output serial}
 The command @command{serial} initializes the serial unit 0 with the
 speed 9600bps. The serial unit 0 is usually called @samp{COM1}, so, if
 you want to use COM2, you must specify @samp{--unit=1} instead. This
-command accepts many other options, so please refer to @ref{serial},
-for more details.
+command accepts many other options, @pxref{serial} for more details.
 
 Without argument or with @samp{--port=auto}, GRUB will attempt to use
 ACPI when available to auto-detect the default serial port and its
@@ -4164,6 +4163,9 @@ Additionally, an MMIO address can be suffixed with:
 @samp{.q} for 64-bit long long word access
 @end itemize
 
+Also, @var{port} can be of the form @samp{pci,XX:XX.X} to indicate a serial
+device exposed on the PCI bus.
+
 @var{speed} is the transmission speed; default is 9600. @var{word} and
 @var{stop} are the number of data bits and stop bits. Data bits must
 be in the range 5-8 and stop bits must be 1 or 2. Default is 8 data
@@ -4179,10 +4181,15 @@ The serial port is not used as a communication channel unless the
 @command{terminal_input} or @command{terminal_output} command is used
 (@pxref{terminal_input}, @pxref{terminal_output}).
 
+Note, valid @var{port} values, excluding IO port addresses, can be found
+by listing terminals with @command{terminal_output}, selecting all names
+prefixed by @samp{serial_} and removing that prefix.
+
 Examples:
 @example
-serial --port=3f8 --speed=9600
+serial --port=0x3f8 --speed=9600
 serial --port=mmio,fefb0000.l --speed=115200
+serial --port=pci,00:16.3 --speed=115200
 @end example
 
 See also @ref{Serial terminal}.