dt-bindings: gpio: add NPCM sgpio driver bindings

Add dt-bindings document for the Nuvoton NPCM7xx sgpio driver

Signed-off-by: Jim Liu <JJLIU0@nuvoton.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Reviewed-by: Paul Menzel <pmenzel@molgen.mpg.de>
Signed-off-by: Bartosz Golaszewski <bartosz.golaszewski@linaro.org>