net/designware: Make DMA burst length configurable and reduce by default

The correct value for this setting can vary across SoCs and boards, so make it
configurable.

Also reduce the default value to 8, which is the same default as used in the
Linux driver.

Signed-off-by: Ian Campbell <ijc@hellion.org.uk>
Cc: Alexey Brodkin <abrodkin@synopsys.com>