x86: qemu: Fix broken multi-core boot

Unfortunately the multi-core boot for QEMU x86 has been broken since
commit 77a5e2d3 ("x86: mp_init: Set up the CPU numbers at the start").

In order to support QEMU x86 multi-core boot, the /cpus node must be
bound before any actual fix up in qemu_cpu_fixup(). This adds the
uclass_get() call to ensure this, just like what was done before.

Fixes: 77a5e2d3 ("x86: mp_init: Set up the CPU numbers at the start")
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>