net: sparx5: use contiguous memory for tx buffers

Currently, the driver uses a linked list for storing the tx buffer
addresses. This requires a good amount of extra bookkeeping code. Ditch
the linked list in favor of tx buffers being in the same contiguous
memory space as the DCB's and the DB's. The FDMA library has a helper
for this - so use that.

The tx buffer addresses are now retrieved as an offset into the FDMA
memory space.

Signed-off-by: Daniel Machon <daniel.machon@microchip.com>
Reviewed-by: Steen Hegelund <Steen.Hegelund@microchip.com>
Reviewed-by: Jens Emil Schulz Østergaard <jensemil.schulzostergaard@microchip.com>
Reviewed-by: Horatiu Vultur <horatiu.vultur@microchip.com>
Signed-off-by: David S. Miller <davem@davemloft.net>