mlxsw: reg: Add MCIA register for cable info access

The MCIA register is used to access the SFP+ and QSFP connector's
EPROM. It will be used to query the cable info.

Signed-off-by: Arkadi Sharshevsky <arkadis@mellanox.com>
Signed-off-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>