ARM: dts: am335x-sl50: add support for DS1339 Real Time Clock

Production hardware will go with the DS1339 RTC chip, so replace the old
for the new one and also add the nIRQ pin to be able to properly wakeup
the system from suspend.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>