mailbox: bcm-flexrm: Use device-managed registration API

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

Signed-off-by: Thierry Reding <treding@nvidia.com>
Acked-by: Scott Branden <scott.branden@broadcom.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>