pinctrl: rockchip: add support for the rk3399

The pinctrl of rk3399 is much different from other's,
especially the 3bits of drive strength.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Reviewed-by: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>