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

This reverts commit 3d76be5b.

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 7a6cb0ab ("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 <johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>

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

@@ -80,3 +80,7 @@ hdmi {
 		status = "okay";
 	};
 };
+
+&rtc {
+	system-power-controller;
+};