x86: qemu: Create separate i440fx and q35 device trees

Although the two qemu-x86 targets (i440fx and q35) share a lot in
common, they still have something that cannot easily handled in one
single device tree). Split to create two dedicated device tree files
and make the i440fx be the default build target.

Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Acked-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/README.x86 b/doc/README.x86
index 0726205..c19f4a0 100644
--- a/doc/README.x86
+++ b/doc/README.x86
@@ -40,7 +40,7 @@
 x86 architecture  --->
 	...
 	(qemu-x86) Board configuration file
-	(qemu-x86) Board Device Tree Source (dts) file
+	(qemu-x86_i440fx) Board Device Tree Source (dts) file
 	(0x01920000) Board specific Cache-As-RAM (CAR) address
 	(0x4000) Board specific Cache-As-RAM (CAR) size
 
@@ -186,6 +186,14 @@
 $ make qemu-x86_defconfig
 $ make all
 
+Note this default configuration will build a U-Boot for the QEMU x86 i440FX
+board. To build a U-Boot against QEMU x86 Q35 board, you can change the build
+configuration during the 'make menuconfig' process like below:
+
+Device Tree Control  --->
+	...
+	(qemu-x86_q35) Default Device Tree for DT control
+
 Test with coreboot
 ------------------
 For testing U-Boot as the coreboot payload, there are things that need be paid