mlxsw: reg: Add Tunneling IPinIP General Configuration Register

The TIGCR register is used for setting up the IPinIP Tunnel
configuration.

Fixes: ee954d1a ("mlxsw: spectrum_router: Support GRE tunnels")
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>