dt-bindings: interrupt-controller: add Andes machine-level software interrupt controller

dt-bindings: interrupt-controller: add Andes machine-level software interrupt controller

Add the DT binding documentation for Andes machine-level software
interrupt controller.

In the Andes platform such as QiLai SoC, the PLIC module is instantiated a
second time with all interrupt sources tied to zero as the software
interrupt controller (PLICSW). PLICSW can generate machine-level software
interrupts through programming its registers.

Acked-by: Conor Dooley <conor.dooley@microchip.com>
Signed-off-by: Ben Zong-You Xie <ben717@andestech.com>
Link: https://lore.kernel.org/r/20250711133025.2192404-5-ben717@andestech.com
Signed-off-by: Arnd Bergmann <arnd@arndb.de>