riscv: Rename Andes PLIC to PLICSW

As PLICSW is used to trigger the software interrupt, we should rename
Andes PLIC configuration and file name to reflect the usage. This patch
also updates PLMT and PLICSW compatible strings to be consistent with
OpenSBI fdt driver.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Rick Chen <rick@andestech.com>