net/macb: GEM DMA configuration register update

Add information to the DMA Configuration Register to
maximize system performance:
- rx/tx packet buffer full memory size
- allow possibility to use INCR16 if supported

Signed-off-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Acked-by: Joachim Eastwood <manabian@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>