ChiliSOM has TPS65217's PWR_EN pin connected to AM335x PMIC_POWER_EN
pin. Processor's PMIC_POWER_EN is controlled by it's internal RTC, hence
RTC subsystem is responsible for proper board poweroff sequence.

This change enables complete poweroff sequence for ChiliBoard, switching
PMIC's state from ACTIVE to SLEEP.
Signed-off-by: Marcin Niestroj <m.niestroj@grinn-global.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>