MAINTAINERS: Add EFI maintainer entry

We're starting to need a channel through which we can funnel EFI
patches to make sure they get merged in a timely fashion. Matthew and
Peter seem happy enough for me to take that bullet.

Signed-off-by: Matt Fleming <matt.fleming@intel.com>
Acked-by: H. Peter Anvin <hpa@zytor.com>
Acked-by: Ingo Molnar <mingo@kernel.org>
Cc: Matthew Garrett <mjg@redhat.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Tony Luck <tony.luck@intel.com>
Cc: Len Brown <len.brown@intel.com>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>