net: dsa: mv88e6xxx: add support for mv88e6250

This adds support for the Marvell 88E6250. I've checked that each
member in the ops-structure makes sense, and basic switchdev
functionality works fine.

It uses the new dual_chip option, and since its port registers start
at SMI address 0x08 or 0x18 (i.e., always sw_addr + 0x08), we need to
introduce a new compatible string in order for the auto-identification
in mv88e6xxx_detect() to work.

The chip has four per port 16-bits statistics registers, two of which
correspond to the existing "sw_in_filtered" and "sw_out_filtered" (but
at offsets 0x13 and 0x10 rather than 0x12 and 0x13, because why should
this be easy...). Wiring up those four statistics seems to require
introducing a STATS_TYPE_PORT_6250 bit or similar, which seems a tad
ugly, so for now this just allows access to the STATS_TYPE_BANK0 ones.

The chip does have ptp support, and the existing
mv88e6352_{gpio,avb,ptp}_ops at first glance seem like they would work
out-of-the-box, but for simplicity (and lack of testing) I'm eliding
this.

Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Rasmus Villemoes <rasmus.villemoes@prevas.dk>
Signed-off-by: David S. Miller <davem@davemloft.net>