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/minnowmax.dts b/arch/x86/dts/minnowmax.dts
index 0e59b18..9527233 100644
--- a/arch/x86/dts/minnowmax.dts
+++ b/arch/x86/dts/minnowmax.dts
@@ -10,6 +10,7 @@
 
 /include/ "skeleton.dtsi"
 /include/ "serial.dtsi"
+/include/ "rtc.dtsi"
 
 / {
 	model = "Intel Minnowboard Max";