ARM: dts: am57xx: cl-som-am57x: dts: add RTC support

Add EM3027 RTC chip support (over I2C4 bus).

Signed-off-by: Dmitry Lifshitz <lifshitz@compulab.co.il>
Acked-by: Igor Grinberg <grinberg@compulab.co.il>
Signed-off-by: Tony Lindgren <tony@atomide.com>