x86: qemu: add a cpu uclass driver for qemu target

Add a cpu uclass driver for qemu. Previously, the qemu target gets cpu
number from board dts files, which are manually created at compile time.
This does not scale when more cpus are assigned to guest as the dts files
must be modified as well.

This patch adds a cpu uclass driver for qemu targets to directly read
online cpu number from firmware.

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