Fix to build DSP Gateway as a module.

After this fix, modules regarding mailbox will be built as follows.

arch/arm/mach-omap[12]/mailbox.o -> mailbox_mach.ko
arch/arm/plat-omap/mailbox.o ->mailbox.ko
Signed-off-by: Toshihiro Kobayashi <toshihiro.kobayashi@nokia.com>