Documentation: networking: dsa: add paragraph for the MRP offload

Add a short summary of the methods that a driver writer must implement
for getting an MRP instance to work on top of a DSA switch.

Cc: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: Vladimir Oltean <vladimir.oltean@nxp.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>