Add a node for the Bus State Controller (BSC) on sh73a0, to which
multiple external devices can be connected.

The BSC is driven by the ZB clock, and located in PM domain A4S.
A reference to the latter will be added later.
Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>