pinctrl: ocelot: Add support for Serval platforms

This patch adds support for Serval pinctrl, using the ocelot driver as
basis. It adds pinconfig support as well, as supported by the
platform.

gclement: Split from a larger patch adding support all platforms in
the same time.

Signed-off-by: Lars Povlsen <lars.povlsen@microchip.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Link: https://lore.kernel.org/r/20201106093118.965152-5-gregory.clement@bootlin.com
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>