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>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>