pwm: mxs: Let device core handle pinctrl

Since commit ab78029e (drivers/pinctrl: grab default handles from device core),
we can rely on device core for handling pinctrl.

So remove devm_pinctrl_get_select_default() from the driver.

Cc: Thierry Reding <thierry.reding@avionic-design.de>
Cc: <linux-kernel@vger.kernel.org>
Signed-off-by: Fabio Estevam <fabio.estevam@freescale.com>
Tested-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Thierry Reding <thierry.reding@gmail.com>