mlxsw: reg: Add SGCR.lag_lookup_pgt_base

Add SGCR.lag_lookup_pgt_base, which is used for configuring the base
address of the LAG table within the PGT table for cases when the driver
is responsible for the table placement.

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