phy: mediatek: tphy: Cleanup and document slew calibration

While it's true that, generally, the T-PHY V3 does not support the
slew calibration process, some minor versions of it actually do,
moreover, some SoCs may not support this even though the version
of the PHY IP does.

The reference clock and rate coefficient parameters are used only
for slew calibration: move those to platform data, then document
and change the checks in hs_slew_rate_calibrate() to perform the
calibration only if:
 - EYE value was not supplied (pre-calculated calibration); and
 - Slew reference clock value is present (not zero); and
 - Slew coefficient is present (not zero).

Moreover, change the probe function to always check if both the
slew reference clock and coefficient properties are present and,
if not, assign the value from platform data (which, as reminder,
if not added means that it's zero!), instead of checking the PHY
IP version.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250623120315.109881-3-angelogioacchino.delregno@collabora.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>

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