sh: Add CPG save/restore code for sh7724 R-standby

Add sh7724 code to save and restore CPG state during
R-standby. Only CPG registers IRDACLKCR and SPUCLKCR
require software save and restore.

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