spi: mt65xx: Add kerneldoc for driver structures

One of the two structures was already partially documented, but not
in kerneldoc format: enhance readability by adding the missing
documentation bits and use kerneldoc.

Signed-off-by: AngeloGioacchino Del Regno <angelogioacchino.delregno@collabora.com>
Link: https://lore.kernel.org/r/20220407114428.167091-8-angelogioacchino.delregno@collabora.com


Signed-off-by: Mark Brown <broonie@kernel.org>