drm/amdgpu: create a new file for doorbell manager

This patch:
- creates a new file for doorbell management.
- moves doorbell code from amdgpu_device.c to this file.

V2:
 - remove doc from function declaration (Christian)
 - remove 'device' from function names to make it consistent (Alex)
 - add SPDX license identifier (Luben)

V3:
 - change license to MIT license(Christian)

Cc: Alex Deucher <alexander.deucher@amd.com>
Cc: Christian Koenig <christian.koenig@amd.com>
Reviewed-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Shashank Sharma <shashank.sharma@amd.com>