phy: mediatek: tphy: Clarify and add kerneldoc to mtk_phy_pdata

As struct mtk_phy_pdata was almost fully documented, transfer
the comments into kerneldoc on top.
While at it, also rewrite the comments to both improve the writing
writing and the actual information in the documentation, and add a
description for the `version` member of the structure.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Neil Armstrong <neil.armstrong@linaro.org>
Link: https://lore.kernel.org/r/20250623120315.109881-2-angelogioacchino.delregno@collabora.com


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