x86: braswell: Disable PUNIT power configuration for B0 stepping

FSP's built-in UPD configuration enables PUNIT power configuration,
but on B0 stepping, this causes CPU hangs in fsp_init(). Disable it.

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