drm/amdgpu: Add nv mailbox irq in soc21

Under virtualization guest needs to receive notification from host to
perform reset in some cases. Add nv mailbox irq in soc21.

Signed-off-by: YuBiao Wang <YuBiao.Wang@amd.com>
Reviewed-by: Hawking Zhang <Hawking.Zhang@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>