bindings: mfd: cros ec: Document vbc EC property

Some EC implementations include a small nvram space used to store
verified boot context data. This boolean property lets us indicate
whether this space is available or not on a specific EC implementation.
Signed-off-by: NEmilio López <emilio.lopez@collabora.co.uk>
Reviewed-by: NJavier Martinez Canillas <javier@osg.samsung.com>
Signed-off-by: NLee Jones <lee.jones@linaro.org>

Documentation/devicetree/bindings/mfd/cros-ec.txt

@@ -34,6 +34,10 @@ Required properties (LPC):
 - compatible: "google,cros-ec-lpc"
 - reg: List of (IO address, size) pairs defining the interface uses
 
+Optional properties (all):
+- google,has-vbc-nvram: Some implementations of the EC include a small
+  nvram space used to store verified boot context data. This boolean flag
+  is used to specify whether this nvram is present or not.
 
 Example for I2C: