dt-bindings: pinctrl: ocelot: Add Serval SoC support

Add the documentation for the Microsemi Serval pinmuxing and gpio
controller.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20201106093118.965152-3-gregory.clement@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>