gpio/mxs: Move Freescale mxs gpio driver to drivers/gpio

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

v2: Removed header file entirely and put struct definition directly
    into driver.  The struct isn't used anywhere else in the kernel.

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