pinctrl: Document "function" + "pins" pinmux binding

Currently the "function" + "groups" combination is the only documented
format for pinmux nodes, although many drivers use "function" + "pins".
Update the generic pinctrl binding to include the "function" + "pins"
combination as well.
Signed-off-by: NAndrew Bresticker <abrestic@chromium.org>
Cc: Rob Herring <robh+dt@kernel.org>
Cc: Pawel Moll <pawel.moll@arm.com>
Cc: Mark Rutland <mark.rutland@arm.com>
Cc: Ian Campbell <ijc+devicetree@hellion.org.uk>
Cc: Kumar Gala <galak@codeaurora.org>
Signed-off-by: NLinus Walleij <linus.walleij@linaro.org>

Documentation/devicetree/bindings/pinctrl/pinctrl-bindings.txt

@@ -133,6 +133,9 @@ pin multiplexing nodes:
 
 function		- the mux function to select
 groups			- the list of groups to select with this function
+			  (either this or "pins" must be specified)
+pins			- the list of pins to select with this function (either
+			  this or "groups" must be specified)
 
 Example:
 
@@ -148,6 +151,10 @@ state_1_node_a {
 		groups = "spi0pins";
 	};
 };
+state_2_node_a {
+	function = "i2c0";
+	pins = "mfio29", "mfio30";
+};
 
 == Generic pin configuration node content ==