x86/efi/bgrt: Move efi-bgrt handling out of arch/x86

x86/efi/bgrt: Move efi-bgrt handling out of arch/x86

Now with open-source boot firmware (EDK2) supporting ACPI BGRT table
addition even for architectures like AARCH64, it makes sense to move
out the 'efi-bgrt.c' file and supporting infrastructure from 'arch/x86'
directory and house it inside 'drivers/firmware/efi', so that this common
code can be used across architectures.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
Signed-off-by: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Matt Fleming <matt@codeblueprint.co.uk>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: linux-efi@vger.kernel.org
Link: http://lkml.kernel.org/r/20170404160245.27812-7-ard.biesheuvel@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>