net: dsa: vsc73xx: add structure descriptions

This commit adds updates to the documentation describing the structures
used in vsc73xx. This will help prevent kdoc-related issues in the future.

Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Reviewed-by: Vladimir Oltean <olteanv@gmail.com>
Signed-off-by: Pawel Dembicki <paweldembicki@gmail.com>
Link: https://lore.kernel.org/r/20240417205048.3542839-6-paweldembicki@gmail.com


Signed-off-by: Jakub Kicinski <kuba@kernel.org>