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/configs/qemu-x86_defconfig b/configs/qemu-x86_defconfig
index 5639cc5..a4c20bd 100644
--- a/configs/qemu-x86_defconfig
+++ b/configs/qemu-x86_defconfig
@@ -13,5 +13,6 @@
 CONFIG_VIDEO_VESA=y
 CONFIG_FRAMEBUFFER_SET_VESA_MODE=y
 CONFIG_FRAMEBUFFER_VESA_MODE_111=y
+CONFIG_DM_RTC=y
 CONFIG_USE_PRIVATE_LIBGCC=y
 CONFIG_SYS_VSNPRINTF=y