dt-bindings: add the grf clock for dw-mipi-dsi

For RK3399, the grf clock should be controlled by dw-mipi-dsi driver,
add the description for this clock.

Acked-by: Rob Herring <robh@kernel.org
Signed-off-by: Chris Zhong <zyw@rock-chips.com>
Reviewed-by: Sean Paul <seanpaul@chromium.org>
Signed-off-by: Sean Paul <seanpaul@chromium.org>
Link: http://patchwork.freedesktop.org/patch/msgid/1490147691-4489-3-git-send-email-zyw@rock-chips.com

Documentation/devicetree/bindings/display/rockchip/dw_mipi_dsi_rockchip.txt

@@ -10,7 +10,7 @@ Required properties:
 - interrupts: Represent the controller's interrupt to the CPU(s).
 - clocks, clock-names: Phandles to the controller's pll reference
   clock(ref) and APB clock(pclk). For RK3399, a phy config clock
-  (phy_cfg) is additional required. As described in [1].
+  (phy_cfg) and a grf clock(grf) are required. As described in [1].
 - rockchip,grf: this soc should set GRF regs to mux vopl/vopb.
 - ports: contain a port node with endpoint definitions as defined in [2].
   For vopb,set the reg = <0> and set the reg = <1> for vopl.