DEVICETREE: Add bindings for the ATH79 DDR controllers

The DDR controller of the ARxxx and AR9xxx families provides an
interface to flush the FIFO between various devices and the DDR.
This is mainly used by the IRQ controller to flush the FIFO before
running the interrupt handler of such devices.

Signed-off-by: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>