staging: board: Add support for devices with complex dependencies

Add support for easy registering of one ore more platform devices that
may:
  - need clocks that are described in DT,
  - be part of a PM Domain.

All these dependencies are optional.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Acked-by: Simon Horman <horms+renesas@verge.net.au>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>