[Blackfin] arch: set default value of DEB_DMA_URGENT to YES to avoid DMA...

[Blackfin] arch: set default value of DEB_DMA_URGENT to YES to avoid DMA aborting caused by conflict between core and DMA

Signed-off-by: Cliff Cai <cliff.cai@analog.com>
Signed-off-by: Bryan Wu <bryan.wu@analog.com>