mlxsw: reg: Add mlxsw_reg_ralue_act_ip2me_tun_pack()

To implement IP-in-IP decapsulation, Spectrum uses LPM entries of type
IP2ME with tunnel validity bit and tunnel pointer set. The necessary
register fields are already available, so add a function to pack the
RALUE as appropriate.

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