dt-bindings: display: rockchip-dsi: document external phys

Some dw-mipi-dsi instances in Rockchip SoCs use external dphys.
In these cases the needs clock will also be generated externally
so these don't need the ref-clock as well.

changes in v5:
- rebased on top of 5.5-rc1
- merged with dsi timing change to prevent ordering conflicts
Signed-off-by: Heiko Stuebner <heiko.stuebner@theobroma-systems.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://patchwork.freedesktop.org/patch/msgid/20191209143130.4553-4-heiko@sntech.de

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

@@ -9,8 +9,9 @@ Required properties:
 - reg: Represent the physical address range of the controller.
 - 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) and a grf clock(grf) are required. As described in [1].
+  clock(ref) when using an internal dphy and APB clock(pclk).
+  For RK3399, a phy config clock (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.
@@ -18,6 +19,8 @@ Required properties:
 - video port 1 for either a panel or subsequent encoder
 
 Optional properties:
+- phys: from general PHY binding: the phandle for the PHY device.
+- phy-names: Should be "dphy" if phys references an external phy.
 - power-domains: a phandle to mipi dsi power domain node.
 - resets: list of phandle + reset specifier pairs, as described in [3].
 - reset-names: string reset name, must be "apb".