MAINTAINERS: update SFI pattern

commit 937f961a ("x86: Move sfi to platform") moved the files, update
the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>