Document: devicetree: add OF documents for lantiq falcon pinctrl

Signed-off-by: John Crispin <blogic@openwrt.org>
Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: linux-kernel@vger.kernel.org