pinctrl: rockchip: Add iomux-route switching support

pinctrl: rockchip: Add iomux-route switching support

On the some rockchip SOCS, some things like rk3399 specific uart2 can use
multiple pins. Somewhere between the pin io-cells and the uart it seems
to have some sort of switch to decide to which pin to actually route the
data.

+-------+    +--------+  /- GPIO4_B0 (pinmux 2)

| uart2 | -- | switch | --- GPIO4_C0 (pinmux 2)

+-------+    +--------+  \- GPIO4_C3 (pinmux 2)
(switch selects one of the 3 pins base on the GRF_SOC_CON7[BIT0, BIT1])

The routing switch is determined by one pin of a specific group to be set
to its special pinmux function. If the pinmux setting is wrong for that
pin the ip block won't work correctly anyway.

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>