pinctrl: rockchip: Add pinctrl support for rk3308

An iomux register contains 8 pins, each of which is represented
by 2 bits, but the register offset is 0x8.

For example, GRF_GPIO0A_IOMUX offset is 0x0, but GRF_GPIO0B_IOMUX
offset is 0x8, the offset 0x4 is reserved.

So add a type IOMUX_8WIDTH_2BIT to calculate offset.

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