MIPS: JZ4740: DTS: Add DMA nodes

Add the devicetree nodes for the DMA core of the JZ4740 SoC, disabled
by default, as currently there are no clients for the DMA driver
(until the MMC driver and/or others get a devicetree node).

Signed-off-by: Paul Cercueil <paul@crapouillou.net>
Tested-by: Mathieu Malaterre <malat@debian.org>
Acked-by: Paul Burton <paul.burton@mips.com>
Signed-off-by: Vinod Koul <vkoul@kernel.org>