drm/amdgpu: add irq domain support

Hardware blocks on the GPU like ACP generate interrupts in
the GPU interrupt controller, but are driven by a separate
driver.  Add an irq domain to the GPU driver so that
blocks like ACP can register a Linux interrupt.

Acked-by: Dave Airlie <airlied@redhat.com>
Acked-by: Christian König <christian.koenig@amd.com>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>