mlxsw: reg: Add Policy-Engine Extended Flexible Action Register

PEFA register is used for accessing an extended flexible action entry
in the central KVD Linear Database.

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