ARM: shmobile: r8a73a4 dtsi: Add memory-controller nodes

Add device nodes for the two DDR Bus State Controllers (DBSC).
The DBSCs are located in the A3BC PM domain, which must not be powered
down, else the system will crash.

A reference to the A3BC PM domain will be added later.

Signed-off-by: Geert Uytterhoeven <geert+renesas@glider.be>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>