pinctrl: at91-pio4: Add pinctrl driver

AT91 PIO4 controller is a combined gpio-controller, pin-mux and
pin-config module. The peripheral's pins are assigned through
per-pin based muxing logic.

The pin configuration is performed on specific registers which
are shared along with the gpio controller. So regard the pinctrl
device as a child of atmel_pio4 device.

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