net: dsa: mv88e6xxx: add cap for IRL

Add capability flags to describe the presence of Ingress Rate Limit unit
registers and an helper function to clear it.

In the meantime, fix a few harmless issues:

  - 6185 and 6095 don't have such registers (reserved)
  - the previous code didn't wait for the IRL operation to complete

Signed-off-by: Vivien Didelot <vivien.didelot@savoirfairelinux.com>
Reviewed-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: David S. Miller <davem@davemloft.net>