mailbox/omap: drop legacy platform device support

OMAP mailbox devices can no longer be created in legacy non-DT
mode, all the relevant code has been cleaned up. The OMAP mailbox
driver will only support devices created from DT going forward,
so drop the legacy platform device support from the driver.

Signed-off-by: Suman Anna <s-anna@ti.com>
Signed-off-by: Jassi Brar <jaswinder.singh@linaro.org>