arm/efi: Allow invocation of arbitrary runtime services

efi_call_runtime() is provided for x86 to be able abstract mixed mode
support.  Provide this for ARM also so that common code work in mixed mode
also.

Suggested-by: Lukas Wunner <lukas@wunner.de>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Signed-off-by: David Howells <dhowells@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>