MIPS: Alchemy: add sysdev for DBDMA PM.

Add a sysdev for DBDMA PM.

Signed-off-by: Manuel Lauss <manuel.lauss@gmail.com>
To: Linux-MIPS <linux-mips@linux-mips.org>
Patchwork: http://patchwork.linux-mips.org/patch/1119/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>