net: dsa: mv88e6xxx: prefix Port Control 2 macros

For implicit namespacing and clarity, prefix the common Port Control 2
Register macros with MV88E6XXX_PORT_CTL2 and the ones which differ
between implementations with a chosen reference model
(e.g. MV88E6095_PORT_CTL2_CPU_PORT_MASK.)

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>