efi/x86: Move 32-bit code into efi_32.c

Now that the old memmap code has been removed, some code that was left
behind in arch/x86/platform/efi/efi.c is only used for 32-bit builds,
which means it can live in efi_32.c as well. So move it over.

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