dmaengine: xgene: Set hardware dependency

The xgene-dma driver is only useful on X-Gene SoC.

Signed-off-by: Jean Delvare <jdelvare@suse.de>
Cc: Rameshwar Prasad Sahu <rsahu@apm.com>
Cc: Loc Ho <lho@apm.com>
Cc: Vinod Koul <vinod.koul@intel.com>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>