net: mvpp2: add support for mii

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>