mlxsw: Add an initial PGT table support

mlxsw: Add an initial PGT table support

The PGT (Port Group Table) table maps an index to a bitmap of local ports
to which a packet needs to be replicated. This table is used for layer 2
multicast and flooding.

In the legacy model, software did not interact with this table directly.
Instead, it was accessed by firmware in response to registers such as
SFTR and SMID. In the new model, the SFTR register is deprecated and
software has full control over the PGT table using the SMID register.

The entire state of the PGT table needs to be maintained in software
because member ports in a PGT entry needs to be reference counted to avoid
releasing entries which are still in use.

Add the following APIs:
1. mlxsw_sp_pgt_{init, fini}() - allocate/free the PGT table.
2. mlxsw_sp_pgt_mid_alloc_range() - allocate a range of MID indexes in PGT.
   To be used by FID code during initialization to reserve specific PGT
   indexes for flooding entries.
3. mlxsw_sp_pgt_mid_free_range() - free indexes in a given range.
4. mlxsw_sp_pgt_mid_alloc() - allocate one MID index in the PGT at a
   non-specific range, just search for free index. To be used by MDB code.
5. mlxsw_sp_pgt_mid_free() - free the given index.

Note that alloc() functions do not allocate the entries in software, just
allocate IDs using 'idr'.

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>