dts: riscv: andes: rename plic-sw node to interrupt controller

The plic-sw is the same hardware as the plic interrupt contoller and should
be used with intc_plic driver instead of separate mbox driver.

Renamed plic-sw node from "mbox: mbox-controller@e6400000" to
"plic_sw: interrupt-controller@e6400000".

Signed-off-by: Jimmy Zheng <jimmyzhe@andestech.com>