pinctrl/lantiq: add pin_config_group_set support

While converting all the boards supported by OpenWrt to OF
I noticed that this feature is missing. Adding it makes the
devicetrees more readable.

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>