dt-bindings: Add vendor prefix for ROCKTECH DISPLAYS LIMITED

Add ROCKTECH DISPLAYS LIMITED (https://rocktech.com.hk) LCD panel
supplier.
Signed-off-by: Guido Günther <agx@sigxcpu.org>
Signed-off-by: Thierry Reding <treding@nvidia.com>
Link: https://patchwork.freedesktop.org/patch/msgid/b8db03faa344c834e6cc81eef1eca154e1a1bd90.1554114302.git.agx@sigxcpu.org

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -335,6 +335,7 @@ ricoh	Ricoh Co. Ltd.
 rikomagic	Rikomagic Tech Corp. Ltd
 riscv	RISC-V Foundation
 rockchip	Fuzhou Rockchip Electronics Co., Ltd
+rocktech	ROCKTECH DISPLAYS LIMITED
 rohm	ROHM Semiconductor Co., Ltd
 ronbo   Ronbo Electronics
 roofull	Shenzhen Roofull Technology Co, Ltd