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

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

 * Add a list to save all mx2 ports references, so that
   mx2_gpio_irq_handler can walk through all interrupt status
   registers

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

 * Change mxc_gpio_init into mxc_gpio_probe function

 * Move "struct mxc_gpio_port" into gpio-mxc.c, as it needs not to
   be public at all, and also make some other cleanup on
   plat-mxc/include/mach/gpio.h at the same time

And the patch then migrates mach-imx and mach-mx5 to the updated
driver by adding corresponding platform devices.

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