arm64/efi: refactor EFI init and runtime code for reuse by 32-bit ARM

This refactors the EFI init and runtime code that will be shared
between arm64 and ARM so that it can be built for both archs.

Reviewed-by: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Will Deacon <will.deacon@arm.com>