It is used to manage the doorbell allocation of mes processes and queues.
Driver calls into process doorbell allocation to get the slice doorbell
for the process, then the doorbell for a queue is allocated from the
process doorbell slice.
Signed-off-by: Jack Xiao <Jack.Xiao@amd.com> Acked-by: Christian König <christian.koenig@amd.com> Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com> Signed-off-by: Alex Deucher <alexander.deucher@amd.com>