Currently, mvpp2 only supports RGMII. This commit adds support for MII.
The description in Marvell's functional specification seems to be wrong.
To enable MII, we need to set GENCONF_CTRL0_PORT3_RGMII, while for RGMII
we need to clear it. This is also how U-Boot handles it.

Signed-off-by: Stefan Eichenberger <eichest@gmail.com>
Reviewed-by: Maxime Chevallier <maxime.chevallier@bootlin.com>
Reviewed-by: Marcin Wojtas <mw@semihalf.com>
Reviewed-by: Russell King (Oracle) <rmk+kernel@armlinux.org.uk>
Link: https://lore.kernel.org/r/20231212141200.62579-1-eichest@gmail.com


Signed-off-by: Paolo Abeni <pabeni@redhat.com>