stmp3xxx_rtc_wdt: Add suspend/resume PM support

There is no conflict with rtc/rtc-stmp3xxx.c parent
because modified registers in PM functions of stmp3xxx_rtc_wdt
are different.

Signed-off-by: Janusz Uzycki <j.uzycki@elproma.com.pl>
Reviewed-by: Guenter Roeck <linux@roeck-us.net>
Signed-off-by: Wim Van Sebroeck <wim@iguana.be>