x86: qemu: remove cpu node in device tree

Remove 'cpu' node in device tree for QEMU targets, and let U-Boot detect
and fix up those information at runtime.

Signed-off-by: Miao Yan <yanmiaobest@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>