dt-bindings: pinctrl: Add bindings for pinctrl-microchip-sgpio driver

This adds DT bindings for the Microsemi/Microchip SGPIO controller,
bindings microchip,sparx5-sgpio, mscc,ocelot-sgpio and
mscc,luton-sgpio.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20201113145151.68900-2-lars.povlsen@microchip.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>