efi/bgrt: Enable ACPI BGRT handling on arm64

Now that the ACPI BGRT handling code has been made generic, we can
enable it for arm64.

Signed-off-by: Bhupesh Sharma <bhsharma@redhat.com>
[ Updated commit log to reflect that BGRT is only enabled for arm64, and added
  missing 'return' statement to the dummy acpi_parse_bgrt() function. ]
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/20170404160245.27812-8-ard.biesheuvel@linaro.org


Signed-off-by: Ingo Molnar <mingo@kernel.org>