fault-inject: add documentation on MMC IO fault injection

Add description on how to enable random fault injection
for MMC IO.

Signed-off-by: Per Forlin <per.forlin@linaro.org>
Acked-by: Akinobu Mita <akinobu.mita@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Chris Ball <cjb@laptop.org>

Documentation/fault-injection/fault-injection.txt

@@ -21,6 +21,11 @@ o fail_make_request
   /sys/block/<device>/make-it-fail or
   /sys/block/<device>/<partition>/make-it-fail. (generic_make_request())
 
+o fail_mmc_request
+
+  injects MMC data errors on devices permitted by setting
+  debugfs entries under /sys/kernel/debug/mmc0/fail_mmc_request
+
 Configure fault-injection capabilities behavior
 -----------------------------------------------
 
@@ -115,7 +120,8 @@ use the boot option:
 
 	failslab=
 	fail_page_alloc=
-	fail_make_request=<interval>,<probability>,<space>,<times>
+	fail_make_request=
+	fail_mmc_request=<interval>,<probability>,<space>,<times>
 
 How to add new fault injection capability
 -----------------------------------------