efi/libstub/arm: default EFI_ARMSTUB_DTB_LOADER to y

Default EFI_ARMSTUB_DTB_LOADER to y to allow the dtb= command
line parameter to function with efi loader.

Required for development purposes and to boot on existing bootloaders
that do not support devicetree provided by the firmware or by the
bootloader.

Fixes: 3d7ee348

 ("efi/libstub/arm: Add opt-in Kconfig option ...")
Signed-off-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>