MTD: MIPS: lantiq: implement OF support

Adds bindings for OF and make use of module_platform_driver for lantiq
based socs.

Signed-off-by: John Crispin <blogic@openwrt.org>
Acked-by: Artem Bityutskiy <artem.bityutskiy@linux.intel.com>
Cc: linux-mtd@lists.infradead.org
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/3811/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>