dt-bindings: Document loongson vendor-prefix

Loongson is a MIPS-compatible processor vendor.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Paul Burton <paulburton@kernel.org>
Cc: linux-mips@vger.kernel.org
Cc: chenhc@lemote.com
Cc: paul.burton@mips.com
Cc: tglx@linutronix.de
Cc: jason@lakedaemon.net
Cc: maz@kernel.org
Cc: linux-kernel@vger.kernel.org
Cc: robh+dt@kernel.org
Cc: mark.rutland@arm.co
Cc: devicetree@vger.kernel.org