tegra20: add pinctrl driver ...

This adds a pinctrl driver for the Tegra 20 line of SoCs. It only
supports the three basic pinconfiguration settings function mux,
tristate control and pullup/down control.

The driver understands the same devicetree bindings as the Linux one,
unimplemented pinconfiguration options will be ignored.

Signed-off-by: Lucas Stach <dev@lynxeye.de>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>