dt-bindings: Add vendor prefix for Ronbo Electronics

Ronbo Electronics manufactures display panels.
Reviewed-by: Paul Kocialkowski <paul.kocialkowski@bootlin.com>
Signed-off-by: Konstantin Sudakov <k.sudakov@integrasources.com>
Signed-off-by: Maxime Ripard <maxime.ripard@bootlin.com>
Link: https://patchwork.freedesktop.org/patch/msgid/a7ab8bd59ad673f88e2c39cf6cb21bd959cf4db4.1550650810.git-series.maxime.ripard@bootlin.com

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -336,6 +336,7 @@ rikomagic	Rikomagic Tech Corp. Ltd
 riscv	RISC-V Foundation
 rockchip	Fuzhou Rockchip Electronics Co., Ltd
 rohm	ROHM Semiconductor Co., Ltd
+ronbo   Ronbo Electronics
 roofull	Shenzhen Roofull Technology Co, Ltd
 samsung	Samsung Semiconductor
 samtec	Samtec/Softing company