pinctrl: Add DT bindings for PIC32 pin control and GPIO

Document the devicetree bindings for PINCTRL and GPIO found on Microchip
PIC32 class devices.

Signed-off-by: Joshua Henderson <joshua.henderson@microchip.com>
Cc: Ralf Baechle <ralf@linux-mips.org>
Acked-by: Rob Herring <robh@kernel.org>
Cc: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>