net: dsa: mv88e6xxx: prefix Port Egress Rate Control macros

For implicit namespacing and clarity, prefix the common Port Egress Rate
Control and Port Egress Rate Control 2 registers macros with
MV88E6XXX_PORT_EGRESS_RATE_CTL1 and MV88E6XXX_PORT_EGRESS_RATE_CTL2.

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>