rtc: stmp3xxx: Add simple binding for the stmp3xxx-rtc

Signed-off-by: Marek Vasut <marex@denx.de>
Cc: Alessandro Zummo <a.zummo@towertech.it>
Cc: devicetree-discuss@lists.ozlabs.org
Cc: Grant Likely <grant.likely@secretlab.ca>
Cc: rtc-linux@googlegroups.com
Acked-by: Rob Herring <rob.herring@calxeda.com>
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>