net/mlx5e: Add API to query/modify SBPR and SBCM registers

To allow users to configure shared receive buffer parameters through
dcbnl callbacks, expose an API to query and modify SBPR and SBCM registers,
which will be used in the upcoming patch.

Signed-off-by: Maher Sanalla <msanalla@nvidia.com>
Reviewed-by: Moshe Shemesh <moshe@nvidia.com>
Signed-off-by: Saeed Mahameed <saeedm@nvidia.com>