net: dsa: mv88e6xxx: prefix Port Control macros

For implicit namespacing and clarity, prefix the common Port Control
Register macros with MV88E6XXX_PORT_CTL0 and the ones which differ
between implementations with a chosen reference model
(e.g. MV88E6185_PORT_CTL0_USE_TAG.)

The reason for CTL0 is to make it clear between the badly named
"Port Control", "Port Control 1" and "Port Control 2" registers.

Document the register and prefer ordered hex masks values for all
Marvell 16-bit registers.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Signed-off-by: David S. Miller <davem@davemloft.net>