The Cross-chip Port Based VLAN Table (PVT) supports two indexing modes,
one using 5-bit for device and 4-bit for port, the other using 4-bit for
device and 5-bit for port, configured via the Global 2 Misc register.

Only 4 bits for the source port are needed when interconnecting 88E6xxx
switch devices since they all support less than 16 physical ports. The
full 5 bits are needed when interconnecting a device with 98DXxxx switch
devices since they support more than 16 physical ports.

Add a mv88e6xxx_pvt_setup helper to set the 4-bit port PVT mode, which
will be extended later to also initialize the PVT content.
Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>