x86: Add some notes for MRC cache with Intel FSP

MRC cache relies on Intel FSP to produce a special GUID that
contains the MRC cache data. Add such information in the
CONFIG_ENABLE_MRC_CACHE help entry.

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