pinctrl: clean up pinconfig-generic documentation

Reword the section of pinctrl-bindings.txt that describes generic
properties that pinctrl bindings may use. The aim is to make the text
clearer, and more explicitly call out the responsibility of individual
bindings that use the generic properties to define which of the
properties are used, and how.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
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>