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>

arch/x86/Kconfig

@@ -274,6 +274,13 @@ config ENABLE_MRC_CACHE
 	  to be used for speeding up boot time on future reboots and/or
 	  power cycles.
 
+	  For platforms that use Intel FSP for the memory initialization,
+	  please check FSP output HOB via U-Boot command 'fsp hob' to see
+	  if there is FSP_NON_VOLATILE_STORAGE_HOB_GUID (asm/fsp/fsp_hob.h).
+	  If such GUID does not exist, MRC cache is not avaiable on such
+	  platform (eg: Intel Queensbay), which means selecting this option
+	  here does not make any difference.
+
 config HAVE_MRC
 	bool "Add a System Agent binary"
 	depends on !HAVE_FSP