pinctrl: at91: add pinctrl driver

pinctrl: at91: add pinctrl driver

AT91 PIO controller is a combined gpio-controller, pin-mux and
pin-config module. The peripheral's pins are assigned through
per-pin based muxing logic.

Each SoC will have to describe the its limitation and pin
configuration via device tree. This will allow to do not need
to touch the C code when adding new SoC if the IP version is
supported.

Signed-off-by: Wenyou Yang <wenyou.yang@atmel.com>
Reviewed-by: Simon Glass <sjg@chromium.org>