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

Add sh7724 code to save and restore RWDT state during
R-standby. Without this patch the watchdog will generate
a reset shortly after resuming from R-standby.

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