mlxsw: reg: Extend mirroring registers with probability rate field

The MPAR and MPAGR registers are used to configure the binding between
the mirroring trigger (e.g., received packet) and the SPAN agent. Add
probability rate field, which will allow us to support sampling by
mirroring to the CPU.

Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>