riscv: andes_plicsw: Fix IPI during OpenSBI invocation

On some AE350 boards, we need to explicitly initialize the priority
registers to a non-zero value so the boot hart can instruct secondary
harts to jump to OpenSBI.

This patch also updates the information about PLICSW.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>