MAINTAINERS: omap: fix regex

Otherwise 'arch/arm/*omap*/foo.c' wouldn't match

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Cc: Joe Perches <joe@perches.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>