dt-binding: pinctrl: Add NPCM7xx pinctrl and GPIO documentation

Added device tree binding documentation for Nuvoton BMC
NPCM750/730/715/705 pinmux and GPIO controller.

Signed-off-by: Tomer Maimon <tmaimon77@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>