x86: mrccache: Allow use before driver model is active

The change to avoid searching the device tree does not work on boards
wich don't have driver model set up this early, for example minnowmax.
Put back the old code (converted to livetree) as a fallback for these
devices. Also update the documentation.

This is tested on minnowmax, link, samus and coral.

Fixes: 87f1084a (x86: Adjust mrccache_get_region() to use livetree)
Signed-off-by: Simon Glass <sjg@chromium.org>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Tested-by: Bin Meng <bmeng.cn@gmail.com> (on Intel minnowmax)