gpio: Add Linus Walleij as gpio co-maintainer

Signed-off-by: Grant Likely <grant.likely@secretlab.ca>
Acked-by: Linus Walleij <linus.walleij@linaro.org>

MAINTAINERS

@@ -2920,6 +2920,7 @@ F:	include/linux/gigaset_dev.h
 
 GPIO SUBSYSTEM
 M:	Grant Likely <grant.likely@secretlab.ca>
+M:	Linus Walleij <linus.walleij@stericsson.com>
 S:	Maintained
 T:	git git://git.secretlab.ca/git/linux-2.6.git
 F:	Documentation/gpio.txt