Add a node for the Bus State Controller (BSC) on r8a73a4, to which
multiple external devices can be connected.

The BSC is driven by the ZB clock, and located in PM domain C4.
A reference to the latter will be added later.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Tested-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: Acked-by: Laurent Pinchart <laurent.pinchart@ideasonboard.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>