pinctrl: zynq: Document DT binding

Add documentation for the devicetree binding for the Zynq pincontroller.

Changes since v1:
 - fix typo
 - add USB related documentation
 - remove 'pinctrl-' prefix for pinctrl sub-nodes
 - update documentation to enforce strict separation of pinmux
   and pinconf nodes
   - update example accordingly

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Tested-by: Andreas Färber <afaerber@suse.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>