pinctrl: mxs: create group for pin config node

The initial mxs pinctrl support, commit 17723111 (pinctrl: add
pinctrl-mxs support) skipped creating group from device tree pin config
node.  Add it to get pin config node work for client device.

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