pinctrl/nomadik: add device tree bindings for db8540

This adds a simple device tree binding for db8540 boards.

Signed-off-by: Gabriel Fernandez <gabriel.fernandez@stericsson.com>
Reviewed-by: Philippe Langlais <philippe.langlais@stericsson.com>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>