drivers: pinctrl: Add pinctrl driver for Microchip PIC32.

In PIC32 pin-controller is a combined gpio-controller, pin-mux and
pin-config module. Remappable peripherals are assigned pins through
per-pin based muxing logic. And pin configuration are performed on
specific port registers which are shared along with gpio controller.
Note, non-remappable peripherals have default pins assigned thus
require no muxing.

Signed-off-by: Purna Chandra Mandal <purna.mandal@microchip.com>
Reviewed-by: Tom Rini <trini@konsulko.com>
Reviewed-by: Daniel Schwierzeck <daniel.schwierzeck@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>