net/designware: align DMA buffer descriptors to D$ line

net/designware: align DMA buffer descriptors to D$ line

It's important to have ability to flush/invalidate each DMA buffer descriptor
individually to prevent incoherency of adjacent BDs.

Signed-off-by: Alexey Brodkin <[email protected]>
Cc: Vineet Gupta <[email protected]>
Cc: Joe Hershberger <[email protected]>
Cc: Vipin Kumar <[email protected]>
Cc: Stefan Roese <[email protected]>
Cc: Shiraz Hashim <[email protected]>
Cc: Albert ARIBAUD <[email protected]>
Cc: Amit Virdi <[email protected]>
Cc: Sonic Zhang <[email protected]>