net/mlx5e: Receive buffer support for DCBX

Add dcbnl's set/get buffer configuration callback that allows user to
set/get buffer size configuration and priority to buffer mapping.

By default, firmware controls receive buffer configuration and priority
of buffer mapping based on the changes in pfc settings. When set buffer
call back is triggered, the buffer configuration changes to manual mode.

The manual mode means mlx5 driver will adjust the buffer configuration
accordingly based on the changes in pfc settings.

ConnectX buffer stride is 128 Bytes. If the buffer size is not multiple
of 128, the buffer size will be rounded down to the nearest multiple of
128.

Signed-off-by: Huy Nguyen <huyn@mellanox.com>
Reviewed-by: Parav Pandit <parav@mellanox.com>
Signed-off-by: Saeed Mahameed <saeedm@mellanox.com>