pinctrl: pinctrl-imx: add imx51 pinctrl driver

ChangeLog v1->v2:
* change PIN_FUNC_ID base in binding doc to 0 from 1.

Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>