devicetree: Add vendor-prefix for Silead Inc.

Silead Inc.specializes in touchscreen technology and got recently
introduced in a binding for the gsl1680 i2c touchscreen.

Therefore add the needed vendor-prefix as well.
Signed-off-by: NHeiko Stuebner <heiko@sntech.de>
Signed-off-by: NRob Herring <robh@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -235,6 +235,7 @@ sigma	Sigma Designs, Inc.
 sii	Seiko Instruments, Inc.
 sil	Silicon Image
 silabs	Silicon Laboratories
+silead	Silead Inc.
 silergy	Silergy Corp.
 siliconmitus	Silicon Mitus, Inc.
 simtek