Simple loopback message test for mailbox.

**NOTE**  Kernel patch required due to mailbox API limitation


Kernel Patch
============
On OMAP3, only one mailbox is created with settings hard-coded for
MPU -> DSP messages.

The pach 'dsp-mailbox-loopack.patch' changes the hard-coded settings
so that the MPU is the receiver of the messages so a simple loopback
test can be done.

An API should be created to the message receiver can be configured at
runtime.


Usage
=====
Check what mailboxes are available on this platform.  For example, on OMAP3:

# ls /sys/devices/platform/omap-mailbox/mbox
dsp 
#

Load the test module using the 'name=' command-line argument
with the name of one of the mailboxes shown.  

Since the kernel patch above changed the 'dsp' mailbox into the
loopback mailbox, use that one:

# insmod ./mytest.ko name=dsp

By default, the test will send 16 test messages with an incremeting number
as the message.  The number of messages can be changed with the 'count=' 
command-line option at module load time.