net/mlx5e: Receive buffer configuration

Add APIs for buffer configuration based on the changes in
pfc configuration, cable len, buffer size configuration,
and priority to buffer mapping.

Note that the xoff fomula is as below
  xoff = ((301+2.16 * len [m]) * speed [Gbps] + 2.72 MTU [B]
  xoff_threshold = buffer_size - xoff
  xon_threshold = xoff_threshold - MTU

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