ARM: sun6i: dt: Add A31 DMA controller to DTSI

Now that we have a DMA driver, we can add the DMA bindings in the DTSI for the
controller and the devices supported that can use DMA.

Signed-off-by: Maxime Ripard <maxime.ripard@free-electrons.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>