x86: Adjust mrccache_get_region() to use livetree

Change the algorithm to first find the flash device then read the
properties using the livetree API. With this change the device is not
probed so this needs to be done in mrccache_save().

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