dmaengine: nbpfaxi: don't build this driver where it cannot be used

Although this driver doesn't have any explicit compile-time architecture
dependencies, it is better not to bloat kernels on those platforms, where
this driver isn't needed, unless a compile test is being performed.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski@gmx.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>