ARM: shmobile: sh73a0 dtsi: Add memory-controller nodes

Add device nodes for the two SDRAM Bus State Controllers.
The SBSCs are located in the A4BC0 resp. A4BC1 PM domains, which must
not be powered down, else the system will crash.

References to the A4BC0 and A4BC1 PM domains will be added later.

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