pinctrl: add generic pins and functions properties

pinctrl bindings can benefit from generic property names that define
which pins a "pin configuration node" affects, and which mux function
to select onto those pins. Document new properties for this purpose so
that other bindings may refer to them.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>