x86: Enable DM RTC support for all x86 boards

Add a RTC node in the device tree to enable DM RTC support.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
(Squashed in 'x86: Fix RTC build error on ivybridge')
diff --git a/arch/x86/dts/rtc.dtsi b/arch/x86/dts/rtc.dtsi
index 93dacd7..1797e04 100644
--- a/arch/x86/dts/rtc.dtsi
+++ b/arch/x86/dts/rtc.dtsi
@@ -1,6 +1,7 @@
 / {
 	rtc {
 		compatible = "motorola,mc146818";
+		u-boot,dm-pre-reloc;
 		reg = <0x70 2>;
 	};
 };