x86: spl: Avoid using init_cache_f_r() from SPL

This function is used by U-Boot proper. It does not set up MTRRs when SPL
is enabled, but we do want this done when it is called from SPL. In fact
it is confusing to use the same function from SPL, since there are quite
a few conditions there.

All init_cache_f_r() really does is commit the MTRRs and set up the cache.
Do this in the SPL's version of this function instead.

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