dt-bindings: mips: Add gardena vendor prefix and board description

This patch adds the vendor prefix for gardena and a short description
including the compatible string for the "GARDENA smart Gateway" based
on the MT7688 SoC.

Signed-off-by: Stefan Roese <sr@denx.de>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Burton <paul.burton@mips.com>
Cc: devicetree@vger.kernel.org
Cc: linux-mips@vger.kernel.org