x86/efi: Prepare GOP handling code for reuse as generic code

In preparation of moving this code to drivers/firmware/efi and reusing
it on ARM and arm64, apply any changes that will be required to make this
code build for other architectures. This should make it easier to track
down problems that this move may cause to its operation on x86.

Note that the generic version uses slightly different ways of casting the
protocol methods and some other variables to the correct types, since such
method calls are not loosely typed on ARM and arm64 as they are on x86.

Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Borislav Petkov <bp@alien8.de>
Cc: David Herrmann <dh.herrmann@gmail.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Peter Jones <pjones@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Will Deacon <will.deacon@arm.com>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/1461614832-17633-17-git-send-email-matt@codeblueprint.co.uk
Signed-off-by: Ingo Molnar <mingo@kernel.org>