clk: rockchip: rk3399: Add SCLK_USB3OTGx_REF support

The SCLK_USB3OTGx_REF clocks is used as reference clock for USB3 block.

Add simple support to get rate of SCLK_USB3OTGx_REF clocks to fix
reference clock period configuration.

Also replace use of 24000000 with the OSC_HZ constant.

Signed-off-by: Jonas Karlman <jonas@kwiboo.se>
Reviewed-by: Quentin Schulz <quentin.schulz@theobroma-systems.com>
Reviewed-by: Kever Yang <kever.yang@rock-chips.com>
diff --git a/drivers/clk/rockchip/clk_rk3399.c b/drivers/clk/rockchip/clk_rk3399.c
index 0b32236..67b2c05 100644
--- a/drivers/clk/rockchip/clk_rk3399.c
+++ b/drivers/clk/rockchip/clk_rk3399.c
@@ -976,7 +976,9 @@
 	case SCLK_UART1:
 	case SCLK_UART2:
 	case SCLK_UART3:
-		return 24000000;
+	case SCLK_USB3OTG0_REF:
+	case SCLK_USB3OTG1_REF:
+		return OSC_HZ;
 	case PCLK_HDMI_CTRL:
 		break;
 	case DCLK_VOP0: