ARM: shmobile: r8a7778 dtsi: Add Bus State Controller node

Add a node for the Bus State Controller (BSC) on r8a7778, to which
multiple external devices can be connected.

Signed-off-by: Ulrich Hecht <ulrich.hecht+renesas@gmail.com>
Acked-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>