pinctrl: rockchip: Split the common set_mux() into per Soc

pinctrl: rockchip: Split the common set_mux() into per Soc

Such as rk3288's pins of pmu_gpio0 are a special feature, which have no
higher 16 writing corresponding bits, use common set_mux() func would
introduce more code, so implement their set_mux() in each Soc's own
file to reduce the size of code.

Signed-off-by: David Wu <david.wu@rock-chips.com>
Signed-off-by: Kever Yang <kever.yang@rock-chips.com>