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>