mailbox: mailbox-test: rename driver as generic test driver

This mailbox-test driver was designed to be generic, so let's remove ST
tag on it and make it generic.

Acked-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>