Documentation: Warn about ME cleaner on Sandy Bridge

Document known issues with 'disabled' ME.

Change-Id: I364f3ed49341523c781eb2f3b41e866f33632a7e
Signed-off-by: Patrick Rudolph <patrick.rudolph@9elements.com>
Reviewed-on: https://review.coreboot.org/c/coreboot/+/32889
Reviewed-by: Felix Held <felix-coreboot@felixheld.de>
Tested-by: build bot (Jenkins) <no-reply@coreboot.org>

Documentation/northbridge/intel/sandybridge/index.md

@@ -6,3 +6,4 @@ This section contains documentation about coreboot on specific Intel "Sandy Brid
 
 - [Native Ram Initialization](nri.md)
 - [RAM initialization feature matrix](nri_features.md)
+- [ME Cleaner](me_cleaner.md)

Documentation/northbridge/intel/sandybridge/me_cleaner.md

@@ -0,0 +1,20 @@
+# ME Cleaner
+It's possible to 'clean' the ME partition within the flash medium as part
+of the build process. While cleaning as much code as possible is removed
+from the ME firmware partition. In this state the ME errors out and doesn't
+operate any more.
+
+**Using a 'cleaned' ME partition may lead to issues and its use should be
+carefully evaulated.**
+
+## Observations with 'cleaned' ME
+
+* Instable LPC bus
+  * SuperIO is malfunctioning
+  * TPM is malfunctioning
+  * Random system shutdowns on high bus activity
+
+## Filing bug reports
+
+Always test with unmodified IFD and ME section before reporting bugs to the
+coreboot project.