omap: mailbox: reorganize headers

Remove kernel.h and module.h since they are not used correctly anyway.

Also, remove device.h since it comes along with platform_device.h
(always will I guess).

Signed-off-by: Felipe Contreras <felipe.contreras@gmail.com>
Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>