soc: mediatek: pwrap: Add kerneldoc for struct pwrap_slv_type

In preparation for adding new members with name abbreviations describe
the struct pwrap_slv_type with kerneldoc to enhance human readability.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Reviewed-by: Alexandre Mergnat <amergnat@baylibre.com>
Link: https://lore.kernel.org/r/20230412131216.198313-4-angelogioacchino.delregno@collabora.com


Signed-off-by: Matthias Brugger <matthias.bgg@gmail.com>