MAINTAINERS: update brcm gpio filename pattern

Commit 23a71fd6 ("dt-bindings: brcm: rationalize Broadcom
documentation naming") renamed the file, update the pattern.

Signed-off-by: Joe Perches <joe@perches.com>
Acked-by: Scott Branden <sbranden@broadcom.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>