clk: rockchip: document hdmi_phy external input for rk3328

The hdmi-phy block inside the soc also loops its pll output back
into the clock controller, so document that already used input clock.

Signed-off-by: Heiko Stuebner <heiko@sntech.de>
Reviewed-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/clock/rockchip,rk3328-cru.txt

@@ -32,6 +32,7 @@ clock-output-names:
  - "clkin_i2s" - external I2S clock - optional,
  - "gmac_clkin" - external GMAC clock - optional
  - "phy_50m_out" - output clock of the pll in the mac phy
+ - "hdmi_phy" - output clock of the hdmi phy pll - optional
 
 Example: Clock controller node: