efi: arm: force use of unsigned type for EFI_PHYS_ALIGN

Ensure that EFI_PHYS_ALIGN is an unsigned type, to prevent spurious
warnings from the type checks in the definition of the max() macro.

Link: https://lore.kernel.org/linux-efi/20201213151306.73558-1-ardb@kernel.org


Signed-off-by: Ard Biesheuvel <ardb@kernel.org>