mlxsw: spectrum: Register switched port analyzers (SPAN) as resource

The switch supports an enhanced switched port analyzer that enables
selecting network traffic for analysis by a network analyzer.

SPAN agents are configured and consumed whenever a tc filter is added
with a mirror action to a new destination. The destination can either be
a physical port (e.g., swp1), a VLAN device or a gretap.

Expose the maximum number of SPAN agents and their current usage to the
user.

Signed-off-by: Danielle Ratson <danieller@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>