omap mailbox: Set a device in logical mbox instance for traceability

With this patch, you'll get the following sysfs directories. This
structure implies that a single platform device, "omap2-mailbox" holds
multiple logical mbox instances. This could be the base to add sysfs
files for each logical mboxes. Then userland application can access a
mbox through sysfs entries if necessary(ex: setting kfifo size
dynamically)

  ~# tree -d -L 2 /sys/devices/platform/omap2-mailbox/
  /sys/devices/platform/omap2-mailbox/
  |-- driver -> ../../../bus/platform/drivers/omap2-mailbox
  |-- mbox
  |   |-- dsp      <- they are each instances of logical mailbox.
  |   |-- ducati
  |   |-- iva2
  |   |-- mbox01
  |   |-- mbox02
  |   |-- mbox03
  |   |-- .....
  |   `-- tesla
  |-- power
  `-- subsystem -> ../../../bus/platform

This was wrongly dropped by:
     commit c7c158e5

Signed-off-by: Hiroshi DOYU <Hiroshi.DOYU@nokia.com>