dmaengine: rcar-dmac: Widen DMA mask to 40 bits

By default, the DMA mask covers only the low 32-bit address space, which
causes SWIOTLB on arm64 to fall back to a bounce buffer for DMA
transfers involving memory outside the 32-bit address space.

The R-Car DMA controller hardware supports a 40-bit address space, hence
widen the DMA mask to 40 bits to actually make use of this feature.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Vinod Koul <vinod.koul@intel.com>