dt-bindings: pinctrl: document Broadcom Northstar pin mux controller

Northstar has mux controller just like Northstar Plus and Northstar2.
It's a bit different though (different registers & pins) so it requires
its own binding.

It's needed to allow other block bindings specify required mux setup.

Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>