dt-bindings: cp110: document the new CP115 pinctrl compatible

A new compatible is going to be used for Armada CP115 pinctrl block,
document it.

Signed-off-by: Grzegorz Jaszczyk <jaz@semihalf.com>
[<miquel.raynal@bootlin.com>: split the documentation out of the
driver commit]
Signed-off-by: Miquel Raynal <miquel.raynal@bootlin.com>
Link: https://lore.kernel.org/r/20190805101607.29811-3-miquel.raynal@bootlin.com


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