net/mlx5: Fix PBMC register mapping

Add reserved mapping to cover all the register in order to avoid setting
arbitrary values to newer FW which implements the reserved fields.

Fixes: 50b4a3c2

 ("net/mlx5: PPTB and PBMC register firmware command support")
Signed-off-by: Aya Levin <ayal@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>