MIPS: SEAD3: Implement OF support.

Activate USE_OF for SEAD-3 platform. Add basic DTS file and convert memory
detection and reservations to use OF.

[ralf@linux-mips.org: Remove unnecessary #ifdef wrapper in generic.h.  Make
<asm/mips-boards/generic.h> inclusion work even without prior
<linux/of_fdt.h> inclusion.]

Signed-off-by: Steven J. Hill <sjhill@mips.com>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/4809/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>