drm/amdgpu: add UAPI for allocating doorbell memory

This patch adds flags for a new gem domain AMDGPU_GEM_DOMAIN_DOORBELL
in the UAPI layer.

V2: Drop 'memory' from description (Christian)

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