mailbox: mailbox-test: Fix module autoload

If the driver is built as a module, autoload won't work because the module
alias information is not filled. So user-space can't match the registered
device with the corresponding module.

Export the module alias information using the MODULE_DEVICE_TABLE() macro.

Before this patch:

$ modinfo drivers/mailbox/mailbox-test.ko | grep alias
$

After this patch:

$ modinfo drivers/mailbox/mailbox-test.ko | grep alias
alias:          of:N*T*Cmailbox-testC*
alias:          of:N*T*Cmailbox-test

Signed-off-by: Javier Martinez Canillas <javier@osg.samsung.com>
Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>