Adds the plumbing to be able to allocate and free RM objects, and
implements RM client/device/subdevice allocation with it.

These will be used by subsequent patches.
Signed-off-by: Ben Skeggs <bskeggs@redhat.com>
Signed-off-by: Dave Airlie <airlied@redhat.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20230918202149.4343-35-skeggsb@gmail.com