mlxsw: spectrum: Initialize PGT table

Initialize PGT table as part of mlxsw_sp_init(). This table will be used
first in the next patch by FID code to set flooding entries, and later by
MDB code to add multicast entries.

Signed-off-by: Amit Cohen <amcohen@nvidia.com>
Reviewed-by: Petr Machata <petrm@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: Paolo Abeni <pabeni@redhat.com>