gpio/mxs: Change gpio-mxs into an upstanding gpio driver

The patch makes necessary changes on gpio-mxs as below to turn it
into an upstanding gpio driver.

 * Clean up the gpio port definition stuff

 * Use readl/writel to replace mach-specific accessors
   __raw_readl/__raw_writel

 * Change mxs_gpio_init into mxs_gpio_probe function

And it then migrates mach-mxs to the updated driver by adding
corresponding platform devices.

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