clk: rockchip: document hdmi_phy external input for rk3328

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>