net: macb: add dma_burst_length config

GEM support higher DMA burst writes/reads than the default (4).
add configuration structure with dma burst length so it could be
applied later to DMA configuration.

Signed-off-by: Ramon Fried <rfried.dev@gmail.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Tested-by: Anup Patel <anup.patel@wdc.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>