ARM: dts: am335x: Add rtc node as system-power-controller

PMIC_PWR_EN pin of RTC on am335x-evm, bone, and boneblack is connected to
PMIC on board, so flag rtc node as system-power-controller to allow
software to poweroff boards.

Signed-off-by: Dave Gerlach <d-gerlach@ti.com>
Signed-off-by: Keerthy <j-keerthy@ti.com>