efi: Add grub_efi_get_ram_base() function for arm64

efi: Add grub_efi_get_ram_base() function for arm64

Since ARM platforms do not have a common memory map, add a helper
function that finds the lowest address region with the EFI_MEMORY_WB
attribute set in the UEFI memory map.

Required for the arm64 efi linux loader to restrict the initrd
location to where it will be accessible by the kernel at runtime.

Signed-off-by: Leif Lindholm <leif.lindholm@linaro.org>
Reviewed-by: Daniel Kiper <daniel.kiper@oracle.com>