phy: hdmi: Add color depth configuration

Extend the HDMI configuration options to allow managing bits per color
channel.  This is required by some PHY drivers such as
rockchip-samsung-hdptx.

Reviewed-by: Dmitry Baryshkov <dmitry.baryshkov@linaro.org>
Signed-off-by: Cristian Ciocaltea <cristian.ciocaltea@collabora.com>
Link: https://lore.kernel.org/r/20250318-phy-sam-hdptx-bpc-v6-2-8cb1678e7663@collabora.com


Signed-off-by: Vinod Koul <vkoul@kernel.org>