x86: Move smbios generation into arch independent directory

We will need the SMBIOS generation function on ARM as well going forward,
so let's move it into a non arch specific location.

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