Document binding, and add the node to tegra*.dtsi.

The driver isn't actually instantiated from this node yet, but the I2S
binding will rely on being able to refer to the APB DMA node using a
phandle.
Signed-off-by: Stephen Warren <swarren@nvidia.com>
Signed-off-by: Olof Johansson <olof@lixom.net>