x86: Update mtrr functions to allow leaving cache alone

At present the mtrr functions disable the cache before making changes and
enable it again afterwards. This is fine in U-Boot, but does not work if
running in CAR (such as we are in SPL).

Update the functions so that the caller can request that caches be left
alone.

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