x86: use invd instead of wbinvd in real mode start code

I do not know why the boot code immediately after the system reset
should write-back the cache content. I think the cache invalidation
should be enough.

I tested this commit with qemu-x86_defconfig, and it worked for me.

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