pinctrl: mxs: skip gpio nodes for group creation

The recent added mxs gpio device tree bindings require gpio nodes
defined under pinctrl node too.  The pinctrl-mxs driver should skip
these node for group parsing and creating.

Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>