drm/amdgpu/cgs: add an interface to access PCI resources

This provides an interface to get access to the base address
of PCI resources (MMIO, DOORBELL, etc.).  Only MMIO and
DOORBELL are implemented right now.  This is necessary to
properly utilize shared drivers on platform devices.  IP
modules can use this interface to get the base address
of the resource and add any additional offset and set the
size when setting up the platform driver(s).

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