Revert "ARM: dts: am335x-boneblack: disable RTC-only sleep"

This reverts commit 3d76be5b933e2a66d85a2f7444e68e99e8a48ad4.

The latest revision of Beaglebone Black does not support RTC-only mode.

To avoid potential hardware damage, RTC-only mode was disabled by
default by commit 7a6cb0abe1aa ("ARM: dts: am335x-boneblack: disable
RTC-only sleep to avoid hardware damage").

Unfortunately, an incorrect fix had already been applied, which instead
of just disabling RTC-only mode, prevents the Beaglebone from powering
down at all.

Revert this patch to fix the power-off regression.

Signed-off-by: Johan Hovold <[e-mail]>
Signed-off-by: Tony Lindgren <[e-mail]>

arch/arm/boot/dts/am335x-boneblack.dts

···
 		status = "okay";
 	};
 };
+
+&rtc {
+	system-power-controller;
+};