net: configure DWMAC DMA by default AXI burst length

Board can define its own AXI burst length to improve DWMAC DMA performance.

v2-changes:
- Avoid write burst len register when the Macro is not defined.

v3-changes:
- Add axi_bus register member to struct eth_dma_regs.

Signed-off-by: Sonic Zhang <sonic.zhang@analog.com>
Acked-by: Joe Hershberger <joe.hershberger@ni.com>