gpio: mxs: Add IRQ_TYPE_EDGE_BOTH support

This patch adds support for IRQ_TYPE_EDGE_BOTH needed for some driver
(gpio-keys).

Inspired from gpio-mxc.c

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Gwenhael Goavec-Merou <gwenhael.goavec-merou@armadeus.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>