riscv: Rename Andes cpu and board names

The current ae350-related defconfigs could also
support newer Andes CPU IP, so modify the names of CPU
from ax25 to andesv5, and board name from ax25-ae350 to ae350.

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