riscv: configs: Add nx25-ae250.h to support RISC-V

Add nx25-ae250 board configuartion options for RISC-V

Signed-off-by: Rick Chen <rick@andestech.com>
Signed-off-by: Rick Chen <rickchen36@gmail.com>
Signed-off-by: Greentime Hu <green.hu@gmail.com>