net/mlx5: Add IFC bits for shared headroom pool PBMC support

net/mlx5: Add IFC bits for shared headroom pool PBMC support

Add hardware interface definitions for shared headroom pool (SHP) in
port buffer management:

- shp_pbmc_pbsr_support: capability bit in PCAM enhanced features
  indicating device support for shared headroom pool in PBMC/PBSR.
- shared_headroom_pool: buffer entry in PBMC register (pbmc_reg_bits)
  for the shared headroom pool configuration, reusing the bufferx
  layout; reduce trailing reserved region accordingly.

Signed-off-by: Alexei Lazar <alazar@nvidia.com>
Signed-off-by: Tariq Toukan <tariqt@nvidia.com>
Link: https://patch.msgid.link/20260309093435.1850724-2-tariqt@nvidia.com
Signed-off-by: Leon Romanovsky <leon@kernel.org>