ARM: shmobile: Add DT bindings for Renesas memory controllers

Add DT bindings for Renesas R-Mobile and SH-Mobile memory controllers.
Currently memory controller device nodes are used only to reference PM
domains, and prevent these PM domains from being powered down, which
would crash the system.

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