cli: Add loopback diagnostic command
cli: Add loopback diagnostic command

The loopback command takes a flag to enable each of the different
loopback modes (-l/--ltssm, -r/--rx-to-tx, -t/--tx-to-rx). Or
-d/--disable can be specified to disable all loopback modes.

If none of these flags are specified the current loopback mode for the
port is printed.

The LTSSM speed can optionally be specified with -s/--speed.

Microsemi/switchtec-user,Microsemi/switchtec-user