drm/amdgpu: add initial support for UMSCH

Add basic data structure, dummy ring functions
and ip functions for UMSCH.

Implement sw_init(ring_init and init_microcodede) and
hw_init(load_microcode), UMSCH can boot up now.

Implement hw_init(ring_start) and hw_fini(ring_stop),
UMSCH is ready for command submission now.

Implement set_hw_resources and add/remove_queue,
UMSCH is ready for scheduling now.

Aggregated doorbell is used to notify UMSCH FW that
there is unmapped queue with corresponding priority level
(e.g., AGDB[0] for Real time band, etc.) is updating its job.

v2: squash together initial patches to avoid breaking the
    build (Alex)

Signed-off-by: Lang Yu <Lang.Yu@amd.com>
Reviewed-by: Leo Liu <leo.liu@amd.com>
Reviewed-by: Veerabadhran Gopalakrishnan <Veerabadhran.Gopalakrishnan@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>