t4240/dts: Enable third elo3 DMA engine support

T4240 has a third DMA engine controller, so add the corresponding DMA
node into the dts file.

Signed-off-by: Chunhe Lan <Chunhe.Lan@freescale.com>
Cc: Scott Wood <scottwood@freescale.com>
[scottwood@freescale.com: reword commit message]
Signed-off-by: Scott Wood <scottwood@freescale.com>