gpio: mxs: Add support for DM/DTS in the mxs_gpio.c driver (DM_GPIO)

This patch adds support for DM/DTS in the mxs_gpio.c driver.
Information regarding per gpio controller pin number is passed via DTS.

Signed-off-by: Lukasz Majewski <lukma@denx.de>
Reviewed-by: Marek Vasut <marex@denx.de>