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: Heiko Stuebner <heiko@sntech.de>
Signed-off-by: Rob Herring <robh@kernel.org>