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>
diff --git a/drivers/pinctrl/rockchip/pinctrl-rockchip.h b/drivers/pinctrl/rockchip/pinctrl-rockchip.h
index 9651e9c..5edc7cb 100644
--- a/drivers/pinctrl/rockchip/pinctrl-rockchip.h
+++ b/drivers/pinctrl/rockchip/pinctrl-rockchip.h
@@ -16,6 +16,7 @@
 #define IOMUX_SOURCE_PMU	BIT(2)
 #define IOMUX_UNROUTED		BIT(3)
 #define IOMUX_WIDTH_3BIT	BIT(4)
+#define IOMUX_8WIDTH_2BIT	BIT(5)
 
 /**
  * Defined some common pins constants