net: dsa: mv88e6xxx: accept phy-mode = "internal" for internal PHY ports

The ethernet-controller dt-schema, mostly pushed forward by Linux, has
the "internal" PHY mode for denoting MAC connections to an internal PHY.

U-Boot may provide device tree blobs where this phy-mode is specified,
so make the Linux driver accept them.

It appears that the current behavior with phy-mode = "internal" was
introduced when mv88e6xxx started reporting supported_interfaces to
phylink. Prior to that, I don't think it would have any issues accepting
this phy-mode.

Fixes: d4ebf12b

 ("net: dsa: mv88e6xxx: populate supported_interfaces and mac_capabilities")
Link: https://lore.kernel.org/linux-arm-kernel/20221205172709.kglithpbhdbsakvd@skbuf/T/
Reported-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Florian Fainelli <f.fainelli@gmail.com>
Tested-by: Tim Harvey <tharvey@gateworks.com> # imx6q-gw904.dts
Link: https://lore.kernel.org/r/20221205194845.2131161-1-vladimir.oltean@nxp.com
Signed-off-by: Paolo Abeni <pabeni@redhat.com>