net: phy: phy_port: Store information about a port's upstream

MII phy_ports are not meant to be connected directly to a link partner.
They are meant to feed into some media converter devices that will
expose an MDI phy_port, so far we only support SFP modules for that.

In the case an MDI phy_port is backed by an MII port (e.g. a SFP
module's port, backed by the SFP cage port), let's keep track of the
port id of the MII port backing it.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Aleksei Sviridkin <f@lex.la>
Reviewed-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Tested-by: Christophe Leroy (CS GROUP) <chleroy@kernel.org>
Signed-off-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Link: https://patch.msgid.link/20261016133107.54054-7-maxime.chevallier@bootlin.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>