gpio: gpio-mxs: Remove unneeded dt checks

mxs is a devicetree only platform, so there is no need to check whether we
are in dt or platform data case.

Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>