gpio/mxc: Move Freescale MXC gpio driver to drivers/gpio

GPIO drivers are getting moved to drivers/gpio for cleanup and
consolidation.  This patch moves the plat-mxc driver.  Follow up
patches will clean it up and make it a fine upstanding gpio driver.

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>