net: fill in MODULE_DESCRIPTION()s for mdio_devres

W=1 builds now warn if module is built without a MODULE_DESCRIPTION().
Add descriptions to the PHY MDIO helpers.

Suggested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Breno Leitao <leitao@debian.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Link: https://lore.kernel.org/r/20240214152741.670178-7-leitao@debian.org


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