arm: dts: am4372: setup rtc as system-power-controller

RTC alarm2 is connected to pmic_en line and hence can be used to control
the pmic enabling/disabling. Hence add the system-power-controller for rtc
node.

Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>