sh-pfc: Add DT support

Support device instantiation through the device tree. The compatible
property is used to select the SoC pinmux information.

Set the gpio_chip device field to the PFC device to enable automatic
GPIO OF support.

Cc: devicetree-discuss@lists.ozlabs.org
Signed-off-by: Laurent Pinchart <laurent.pinchart+renesas@ideasonboard.com>
Acked-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>