mailbox: mailbox-test: fix the compatible string

Underscores are usually forbidden in the compatible strings. So lets
remove it before the first users of this is seen.

Acked-by: Rob Herring <robh@kernel.org>
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>