pinctrl: rockchip: Special treatment for RK3288 gpio0 pins' iomux

RK3288 pmu_gpio0 iomux setting have no higher 16 writing corresponding
bits, need to read before write the register.

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