riscv: defconfig: Add nx25-ae250 defconfig to support RISC-V

Add nx25-ae250 default configuration 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>