dt-bindings: interrupt-controller: Andestech Internal Vector Interrupt Controller

This patch adds an irqchip driver document for the Andestech Internal Vector
Interrupt Controller.

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Greentime Hu <greentime@andestech.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Acked-by: Arnd Bergmann <arnd@arndb.de>