dt-bindings: gnss: add u-blox,neo-6m compatible

Add compatible for u-blox NEO-6M GPS module.
Signed-off-by: Ondrej Jirman <megous@megous.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Johan Hovold <johan@kernel.org>

Documentation/devicetree/bindings/gnss/u-blox.txt

@@ -9,6 +9,7 @@ Required properties:
 
 - compatible	: Must be one of
 
+			"u-blox,neo-6m"
 			"u-blox,neo-8"
 			"u-blox,neo-m8"