sh: Rework SuperH Mobile sleep mode code

Rework the SuperH Mobile sleep code from including
board specific code to allowing each board to provide
pre/post code snippets. These snippets should contain
sdram management code to enter and leave self-refresh.

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