MIPS: lantiq: convert dma to platform driver

Add code to make the dma driver load as a platform device from the devicetree.

Signed-off-by: John Crispin <blogic@openwrt.org>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3824/


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