sh: sh7724 ddr self-refresh changes

This patch updates the SuperH Mobile sleep assembly code with
support for DBSC memory controller found in the sh7724 processor.

Without this fix the memory hooked up to the sh7724 processor
will never enter self-refresh mode before suspending to ram. The
effect of this is that the memory contents most likeley will be
lost upon resume which may or may not be what you want.

Signed-off-by: Magnus Damm <damm@igel.co.jp>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>