mlxsw: core: Add EMAD string TLV

Add EMAD string TLV, an ASCII string the driver can receive from the
firmware in case of an error.

Signed-off-by: Shalom Toledo <shalomt@mellanox.com>
Acked-by: Jiri Pirko <jiri@mellanox.com>
Signed-off-by: Ido Schimmel <idosch@mellanox.com>
Signed-off-by: David S. Miller <davem@davemloft.net>