x86: Use mtrr_commit() with FSP2

With FSP2 we use MTRRs in U-Boot proper even though the 32-bit init
happens in TPL. Enable this, using a variable to try to make the
conditions more palatable.

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