sh: Add MMU and Cache handling sleep mode code

Add MMU and cache handling functionality to the SuperH Mobile
sleep code. The MMU and cache registers are saved and restored.
The MMU is disabled and the cache is flushed and disabled before
entering sleep modes if the SUSP_SH_MMU flag is set. This flag
should be set in the case of R-standby and most likely for future
U-standby support as well.

Signed-off-by: Magnus Damm <damm@opensource.se>
Signed-off-by: Paul Mundt <lethal@linux-sh.org>