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

This reverts commit 0d67fac2.

As real hardware testing (*) shows the above mentioned commit
breaks U-Boot on it. Revert for the upcoming release. We may get
more information in the future and optimize the code accordingly.

(*) on Intel Edison board.

Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>
Acked-by: Bin Meng <bmeng.cn@gmail.com>
[bmeng: fix a typo in the commit message]
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>