This mailbox-test driver was designed to be generic, so let's remove ST
tag on it and make it generic.
Acked-by: NLee Jones <lee.jones@linaro.org>
Signed-off-by: NSudeep Holla <sudeep.holla@arm.com>
Signed-off-by: NJassi Brar <jaswinder.singh@linaro.org>