dmaengine: rcar-dmac: Add dma-channel-mask property support

This patch adds dma-channel-mask property support not to reserve
some DMA channels for some reasons. (for example: a heterogeneous
CPU uses it.)

Signed-off-by: Yoshihiro Shimoda <yoshihiro.shimoda.uh@renesas.com>
Reviewed-by: Simon Horman <horms+renesas@verge.net.au>
Reviewed-by: Geert Uytterhoeven <geert+renesas@glider.be>
Link: https://lore.kernel.org/r/1568010892-17606-5-git-send-email-yoshihiro.shimoda.uh@renesas.com
Signed-off-by: Vinod Koul <vkoul@kernel.org>