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>