i2c: add binding to mark a bus as supporting SMBus-Alert

Since SMBus-Alert is an optional feature of SMBUS which
requires an additional pin, the smbus binding cannot be
used to indicate its support.

Add an additional smbus-alert binding specific for it and
update the description text of smbus to avoid mentioning
SMBus-Alert

Signed-off-by: Alain Volmat <alain.volmat@foss.st.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Wolfram Sang <wsa@kernel.org>

Documentation/devicetree/bindings/i2c/i2c.txt

@@ -89,8 +89,11 @@ wants to support one of the below features, it should adapt these bindings.
 
 - smbus
 	states that additional SMBus restrictions and features apply to this bus.
-	Examples of features are SMBusHostNotify and SMBusAlert. Examples of
-	restrictions are more reserved addresses and timeout definitions.
+	An example of feature is SMBusHostNotify. Examples of restrictions are
+	more reserved addresses and timeout definitions.
+
+- smbus-alert
+	states that the optional SMBus-Alert feature apply to this bus.
 
 Required properties (per child device)
 --------------------------------------