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 <[e-mail]>
Reviewed-by: Rob Herring <[e-mail]>

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

···
  - "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: