pinctrl: imx-iomux-v3: only register pinctrl when device node is available
The pinctrl support is devicetree only, so only register it
when we actually have a devicetree. Otherwise we crash boards
using the iomux-v3 from platform code in pinctrl_register().
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>