pinctrl: rmobile: Add pinconf to Renesas RCar pincontrol driver

Add pinconf support to the PFC driver, so that it can handle DT
props bias-disable, bias-pull-up, bias-pull-down, drive-strength
and power-source.

Signed-off-by: Marek Vasut <marek.vasut+renesas@gmail.com>
Cc: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>
Signed-off-by: Nobuhiro Iwamatsu <iwamatsu@nigauri.org>