pinctrl: rockchip: Add iomux-route switching support for rk3288

The rk3288 also has one function that can be routed to one of two pins,
the hdmi cec functionality can use either gpio7c0 or gpio7c7.
So add the route switching support for it.

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