ARM: mx51: Add pinctrl_provide_dummies()

commit a2aa65a3 (ARM: imx: enable pinctrl dummy states) missed to add
pinctrl_provide_dummies() for mx51.

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