dt: pinctrl: Document device tree binding

The core pin controller bindings define:
* The fact that pin controllers expose pin configurations as nodes in
  device tree.
* That the bindings for those pin configuration nodes is defined by the
  individual pin controller drivers.
* A standardized set of properties for client devices to define numbered
  or named pin configuration states, each referring to some number of the
  afore-mentioned pin configuration nodes.
* That the bindings for the client devices determines the set of numbered
  or named states that must exist.

Signed-off-by: Stephen Warren <swarren@nvidia.com>
Acked-by: Shawn Guo <shawn.guo@linaro.org>
Acked-by: Tony Lindgren <tony@atomide.com>
Acked-by: Linus Walleij <linus.walleij@linaro.org>
Acked-by: Simon Glass <sjg@chromium.org>
Acked-by: Dong Aisheng <dong.aisheng@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>