Add rule on maximum platform boots in the Trial state.

The UEFI implementation must track the platform boots in the Trial state
and revert to the previously working bank once the maximum threshold is
crossed.

source/chapter2-uefi.rst

@@ -133,3 +133,13 @@ The FW vendor signature should be placed before the FW image as is described in
 
 The FW images should be authenticated before being written to the FW store or before being
 allowed to execute on the platform.
+
+
+
+Maximum Trial platform boots
+----------------------------
+
+The UEFI implementation must keep a count of the consecutive platform boots in
+the Trial state [FWU]_.  If the number of consecutive platform boot in the
+Trial state exceeds a platform defined value of *max_trial_boots* then the UEFI
+implementation must revert the FW to the previous working bank [FWU]_.