dmaengine: add device tree binding documentation for the nbpfaxi driver

The nbpfaxi dmaengine driver doesn't define any new bindings, it only
uses standard dmaengine bindings and defines 3 flags for the 3rd parameter
of the "dmas" property.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>