phy: mediatek: phy-mtk-hdmi-mt8195: Fix TMDS clk bit ratio setting

The comment in the mtk_phy_tmds_clk_ratio() function clearly and
correctly explains that the TMDS ratio has to be 1/10 for data
rates under 3.4Gbps, and 1/40 over that.

Unfortunately though, the TXC_DIV register setting was wrong, as
in value 3 means to divide by 8 and, in order to achieve the in
spec 1/40 (tmds) data rate, this has to divide by 4 instead!

Add definitions for the TXC_DIV register values clearly explaining
the meanings (DIV2, DIV4, DIV8), and program the correct, DIV 4,
value to the register in mtk_phy_tmds_clk_ratio().

This fixes out of spec clocking and, with this change, SoCs using
the MT8195 class HDMI PHYs can now successfully be configured to
output 3840x2160@60Hz over HDMI.

Fixes: 45810d486bb4 ("phy: mediatek: add support for phy-mtk-hdmi-mt8195")
Reviewed-by: Manivannan Sadhasivam <manivannan.sadhasivam@oss.qualcomm.com>
Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://patch.msgid.link/20260911074015.9994-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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