dt-bindings: pinctrl: mvebu: Document bindings for AC5

Add JSON schema for marvell,ac5-pinctrl present on the Marvell 98DX2530
SoC.

Signed-off-by: Chris Packham <chris.packham@alliedtelesis.co.nz>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20220414233055.586962-2-chris.packham@alliedtelesis.co.nz
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>