Refactor arm-uboot code to make it genereic.

arm-coreboot startup code can be very similar to arm-uboot but current code has
U-Boot specific references. So split U-Boot part from generic part.

include/grub/arm/startup.h

@@ -0,0 +1,16 @@
+#ifndef GRUB_STARTUP_CPU_HEADER
+#define GRUB_STARTUP_CPU_HEADER
+
+struct grub_arm_startup_registers
+{
+  /* registers 0-11 */
+  /* for U-boot r[1] is machine type */
+  /* for U-boot r[2] is boot data */
+  grub_uint32_t r[12];
+  grub_uint32_t sp;
+  grub_uint32_t lr;
+};
+
+extern struct grub_arm_startup_registers grub_arm_saved_registers;
+
+#endif