mlxsw: reg: Add "desc" field to SBPR

SBPR, or Shared Buffer Pools Register, configures and retrieves the shared
buffer pools and configuration. The desc field determines whether the
configuration relates to the byte pool or the descriptor pool.

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