mailbox: platform-mhu: Use device-managed registration API

Get rid of some boilerplate driver removal code by using the newly added
device-managed registration API.

Acked-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>