i2c: add 'single-master' property to generic bindings

It is useful to know if we are the only master on a given bus. Because
this is a HW description of the bus, add it to the generic bindings.

Signed-off-by: Wolfram Sang <wsa+renesas@sang-engineering.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -70,7 +70,12 @@ wants to support one of the below features, it should adapt these bindings.
 - 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.
+	all the time, for example. Can not be combined with 'single-master'.
+
+- single-master
+	states that there is no other master active on this bus. The OS can use
+	this information to detect a stalled bus more reliably, for example.
+	Can not be combined with 'multi-master'.
 
 Required properties (per child device)
 --------------------------------------