net: dsa: mv88e6xxx: add cap for MGMT Enables bits

Some switches provide a Rsvd2CPU mechanism used to choose which of the
16 reserved multicast destination addresses matching 01:80:c2:00:00:0x
should be considered as MGMT and thus forwarded to the CPU port.

Other switches extend this mechanism to also configure as MGMT the
additional 16 reserved multicast addresses matching 01:80:c2:00:00:2x.

This mechanism is exposed via two registers in Global 2, and an Rsvd2CPU
enable bit in the management register.

Newer chip (such as 88E6390) has replaced these registers with a new
indirect MGMT mechanism in Global 1.

The patch adds two MV88E6XXX_FLAG_G2_MGMT_EN_{0,2}X flags to describe
the presence of these Global 2 registers. If 88E6390 support is added, a
MV88E6XXX_FLAG_G1_MGMT_CTRL flag will be needed to setup Rsvd2CPU.

Note: all switches still support in parallel the ATU Load operation with
an MGMT Entry State to forward such frames in a less convenient way.

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>