multiple-os: don't disable timer6

It seems kernel will use it

Change-Id: Idcd61cdf4f3ee8e038883fc0e8699b422fd48ff5
Signed-off-by: Jacob Chen <jacob2.chen@rock-chips.com>

arch/arm/boot/dts/rk3288.dtsi

@@ -289,7 +289,6 @@
 		clock-frequency = <24000000>;
 	};
 
-#if 0
 	timer: timer@ff810000 {
 		compatible = "rockchip,rk3288-timer";
 		reg = <0xff810000 0x20>;
@@ -297,7 +296,6 @@
 		clocks = <&xin24m>, <&cru PCLK_TIMER>;
 		clock-names = "timer", "pclk";
 	};
-#endif
 
 	display-subsystem {
 		compatible = "rockchip,display-subsystem";