This adds support to the kernel proper for booting via UEFI. It shares
most of the code with arm64, so this patch mostly just wires it up for
use with ARM.

Note that this does not include the EFI stub, it is added in a subsequent
patch.
Tested-by: NRyan Harkin <ryan.harkin@linaro.org>
Reviewed-by: NMatt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: NArd Biesheuvel <ard.biesheuvel@linaro.org>