i2c: document binding for multi-master case

We need this binding because some I2C master drivers will need to adapt
their PM settings for the arbitration circuitry.
Acked-by: NGeert Uytterhoeven <geert+renesas@glider.be>
Acked-by: NRob Herring <robh@kernel.org>
Signed-off-by: NWolfram Sang <wsa+renesas@sang-engineering.com>
Signed-off-by: NWolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -54,6 +54,11 @@ wants to support one of the below features, it should adapt the bindings below.
 	"irq" and "wakeup" names are recognized by I2C core, other names are
 	left to individual drivers.
 
+- multi-master
+	states that there is another master active on this bus. The OS can use
+	this information to adapt power management to keep the arbitration awake
+	all the time, for example.
+
 - wakeup-source
 	device can be used as a wakeup source.