dt-bindings: interrupt-controller: Add Loongson HTVEC

Add binding for Loongson-3 HyperTransport Interrupt Vector Controller.

Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Marc Zyngier <maz@kernel.org>
Link: https://lore.kernel.org/r/20200528152757.1028711-3-jiaxun.yang@flygoat.com