net: dsa: mt7530: Add EN7581 support

Introduce support for the DSA built-in switch available on the EN7581
development board. EN7581 support is similar to MT7988 one except
it requires to set MT7530_FORCE_MODE bit in MT753X_PMCR_P register
for on cpu port.

Tested-by: Benjamin Larsson <benjamin.larsson@genexis.eu>
Signed-off-by: Lorenzo Bianconi <lorenzo@kernel.org>
Reviewed-by: Arınç ÜNAL <arinc.unal@arinc9.com>
Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
Signed-off-by: David S. Miller <davem@davemloft.net>