gpio/mxc: convert gpio-mxc to use generic irq chip

The patch converts gpio-mxc driver to use generic irq chip.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Sascha Hauer <s.hauer@pengutronix.de>
Signed-off-by: Grant Likely <grant.likely@secretlab.ca>