MIPS: JZ4770: DTS: Add DMA nodes

Add the two devicetree nodes for the two DMA cores of the JZ4770 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>