xen/common: Move Arm's bootfdt.c to common

xen/common: Move Arm's bootfdt.c to common

Move Arm's bootfdt.c to xen/common so that it can be used by other
device tree architectures like PPC and RISCV.

Remove stubs for process_shm_node() and early_print_info_shmem()
from $xen/arch/arm/include/asm/static-shmem.h.
These stubs are removed to avoid introducing them for architectures
that do not support CONFIG_STATIC_SHM.
The process_shm_node() stub is now implemented in common code to
maintain the current behavior of early_scan_code() on ARM.
The early_print_info_shmem() stub is only used in early_print_info(),
so it's now guarded with #ifdef CONFIG_STATIC_SHM ... #endif.

Suggested-by: Julien Grall <julien@xen.org>
Signed-off-by: Shawn Anastasio <sanastasio@raptorengineering.com>
Signed-off-by: Oleksii Kurochko <oleksii.kurochko@gmail.com>
Acked-by: Julien Grall <jgrall@amazon.com>