of: Add vendor prefix for Hardkernel

Add Hardkernel Co., Ltd. to the list of device tree vendor prefixes.
Signed-off-by: Krzysztof Kozlowski <k.kozlowski.k@gmail.com>
Reviewed-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Kukjin Kim <kgene@kernel.org>

Documentation/devicetree/bindings/vendor-prefixes.txt

@@ -90,6 +90,7 @@ gumstix	Gumstix, Inc.
 gw	Gateworks Corporation
 hannstar	HannStar Display Corporation
 haoyu	Haoyu Microelectronic Co. Ltd.
+hardkernel	Hardkernel Co., Ltd
 himax	Himax Technologies, Inc.
 hisilicon	Hisilicon Limited.
 hit	Hitachi Ltd.