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 <abrodkin@synopsys.com>

Cc: Vineet Gupta <vgupta@synopsys.com>
Cc: Joe Hershberger <joe.hershberger@ni.com>
Cc: Vipin Kumar <vipin.kumar@st.com>
Cc: Stefan Roese <sr@denx.de>
Cc: Shiraz Hashim <shiraz.hashim@st.com>
Cc: Albert ARIBAUD <albert.u.boot@aribaud.net>
Cc: Amit Virdi <amit.virdi@st.com>
Cc: Sonic Zhang <sonic.zhang@analog.com>