ARM: zynq: DT: Add DDRC node

Add the DDR controller to the Zynq devicetree.

Signed-off-by: Soren Brinkmann <soren.brinkmann@xilinx.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Michal Simek <michal.simek@xilinx.com>