mailbox: omap: Remove device class

The driver currently creates a new device class "mbox". Then for each
mailbox adds a device to that class. This class provides no file
operations provided for any userspace users of this device class.
It may have been extended to be functional in our vendor tree at
some point, but that is not the case anymore, nor does it matter
for the upstream tree.

Remove this device class and related functions and variables.
This also allows us to switch to module_platform_driver() as
there is nothing left to do in module_init().

Signed-off-by: Andrew Davis <afd@ti.com>
Signed-off-by: Jassi Brar <jassisinghbrar@gmail.com>