sandbox: fix cpu property in test.dts for pytest

When I tried to run some new efi tests with pytest, efi_smbios_register()
triggered a segmentation fault. Here is the location where it happened:
efi_init_obj_list()
  efi_smbios_register()
    write_smbios_table()
      smbios_write_type4()
        smbios_write_type4_dm()

where dev_get_parent_platdata() should return a pointer to struct
cpu_platdata, but it is actually NULL because any cpu device on
sandbox is attached to "root_driver."

With this patch, this issue will be fixed by moving all the definitions
of cpus under "cpus" node so that they have a "cpu_bus" parent.

Signed-off-by: AKASHI Takahiro <takahiro.akashi@linaro.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Simon Glass <sjg@chromium.org>