platform/surface: aggregator_registry: Add base device hub

platform/surface: aggregator_registry: Add base device hub

The Surface Book 3 has a detachable base part. While the top part
(so-called clipboard) contains the CPU, touchscreen, and primary
battery, the base contains, among other things, a keyboard, touchpad,
and secondary battery.

Those devices do not react well to being accessed when the base part is
detached and should thus be removed and added in sync with the base. To
facilitate this, we introduce a virtual base device hub, which
automatically removes or adds the devices registered under it.

Signed-off-by: Maximilian Luz <luzmaximilian@gmail.com>
Link: https://lore.kernel.org/r/20210212115439.1525216-3-luzmaximilian@gmail.com
Signed-off-by: Hans de Goede <hdegoede@redhat.com>