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

Fixes: http://bugs.elinux.org/issues/143

Entering RTC-only sleep is only properly supported on early prototypes series
(pre-A6) of the BeagleBone Black.  Since rev (A6A), which include all production
versions, it is not support at due to.

(rev A6) enable of the 3v3b regulator moved from LDO2 to LDO4 (3v3a)
side-effect: 3v3b rail remains on in sleep-mode (also in off-mode when battery-powered)

(rev A6A) am335x vdds supply moved from LDO3 to LDO1
side-effect: vdds remains supplied in sleep-mode

Reported-by: Matthijs van Duin <matthijsvanduin@gmail.com>
Tested-by: Matthijs van Duin <matthijsvanduin@gmail.com>
Signed-off-by: Robert Nelson <robertcnelson@gmail.com>
Cc: Tony Lindgren <tony@atomide.com>
Cc: Felipe Balbi <balbi@ti.com>
Cc: Johan Hovold <johan@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>

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

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