rtc: omap: use of_device_is_system_power_controller function

Use of_device_is_system_power_controller instead of manually reading
the system-power-controller property from the device tree node.

Reviewed-by: Johan Hovold <johan@kernel.org>
Signed-off-by: Keerthy <j-keerthy@ti.com>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>