dt-bindings: gnss: add lna-supply property

Add lna-supply property.
Signed-off-by: Andreas Kemnade <andreas@kemnade.info>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Documentation/devicetree/bindings/gnss/gnss.txt

@@ -17,6 +17,7 @@ Required properties:
 		  represents
 
 Optional properties:
+- lna-supply	: Separate supply for an LNA
 - enable-gpios	: GPIO used to enable the device
 - timepulse-gpios	: Time pulse GPIO