net: dsa: mt7530: introduce driver for MT7988 built-in switch

Add driver for the built-in Gigabit Ethernet switch which can be found
in the MediaTek MT7988 SoC.

The switch shares most of its design with MT7530 and MT7531, but has
it's registers mapped into the SoCs register space rather than being
connected externally or internally via MDIO.

Introduce a new platform driver to support that.

Signed-off-by: Daniel Golle <daniel@makrotopia.org>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>