MIPS: Cobalt: Do not build MTD platform device registration code as module.

If CONFIG_MTD_PHYSMAP is set to m, the Cobalt mtd.ko module might get
unloaded while the drivers/mtd modules are still loaded resulting in
stale references to the destroyed platform_device instance.

Anyway, platform devices should always be registered indicated what
devices are present, _not_ what drivers have been configured.

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