Documentation: DT: Fix a typo in the filename "lantiq,<chip>-pinumx.txt"

To fix it, use the chance to rename according the compatible string,
which is "lantiq,pinctrl-falcon" and  "lantiq,pinctrl-xway"
Signed-off-by: Thomas Langer <thomas.langer@lantiq.com>
Cc: John Crispin <blogic@openwrt.org>
Signed-off-by: Rob Herring <robh@kernel.org>