[Why]
In DC we want to wait for the INBOX0 HW Lock command to ACK before
continuing. This is to ensure that the lock has been successfully
acquired before programming HW in DC.

[How]
Add interfaces to send messages on INBOX0, poll for their completation
and clear the ack.
Reviewed-by: Nicholas Kazlauskas <Nicholas.Kazlauskas@amd.com>
Acked-by: Anson Jacob <Anson.Jacob@amd.com>
Tested-by: Daniel Wheeler <daniel.wheeler@amd.com>
Signed-off-by: Alvin Lee <Alvin.Lee2@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>