mlxsw: spectrum_buffers: Move here the new headroom code

Move all the headroom code to the spectrum_buffers module, where it
belongs.

Rename mlxsw_sp_pg_buf_threshold_get() and mlxsw_sp_pg_buf_pack() to
..._hdroom_... to match the naming convention of the new headroom code.

Signed-off-by: Petr Machata <petrm@nvidia.com>
Reviewed-by: Jiri Pirko <jiri@nvidia.com>
Signed-off-by: Ido Schimmel <idosch@nvidia.com>
Signed-off-by: David S. Miller <davem@davemloft.net>