drivers/mrc_cache: Make CACHE_MRC_SETTINGS not selectable

Don't allow the user to select this manually, since it doesn't build
on platforms that don't use it.

Don't set the bool value so that it doesn't show as not selected in
the .config file of platforms that don't use this.

Change-Id: Icf026a297204868d485be270ccee7e0bec0ac73b
Signed-off-by: Arthur Heymans <arthur@aheymans.xyz>
Reviewed-on: https://review.coreboot.org/22933
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>
Reviewed-by: Martin Roth <martinroth@google.com>

src/drivers/mrc_cache/Kconfig

@@ -1,6 +1,7 @@
 config CACHE_MRC_SETTINGS
-	bool "Save cached MRC settings"
-	default n
+	bool
+	help
+	  Save cached MRC settings
 
 if CACHE_MRC_SETTINGS