ARM: dts: am3517-som: Add Seiko Instruments RTC s35390a

The SOM used in the am3517-evm has a s35390a onboard.  This
patch allows the s35390a to be used as the RTC.

Signed-off-by: Derald D. Woods <woods.technical@gmail.com>
Signed-off-by: Adam Ford <aford173@gmail.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>