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

Add nx25-ae250 default configuration for RISC-V
Signed-off-by: NRick Chen <rick@andestech.com>
Signed-off-by: NRick Chen <rickchen36@gmail.com>
Signed-off-by: NGreentime Hu <green.hu@gmail.com>

configs/nx25-ae250_defconfig

+CONFIG_RISCV=y
+CONFIG_TARGET_NX25_AE250=y
+CONFIG_DEFAULT_DEVICE_TREE="ae250"
+CONFIG_FIT=y
+CONFIG_BOOTDELAY=3
+CONFIG_SYS_PROMPT="RISC-V # "
+CONFIG_CMD_MMC=y
+CONFIG_CMD_SF=y
+CONFIG_CMD_SF_TEST=y
+# CONFIG_CMD_SETEXPR is not set
+CONFIG_CMD_DHCP=y
+CONFIG_CMD_PING=y
+CONFIG_CMD_CACHE=y
+CONFIG_CMD_EXT2=y
+CONFIG_CMD_FAT=y
+CONFIG_OF_CONTROL=y
+CONFIG_ENV_IS_IN_SPI_FLASH=y
+CONFIG_NET_RANDOM_ETHADDR=y
+CONFIG_DM=y
+CONFIG_CLK=y
+CONFIG_MMC=y
+CONFIG_DM_SPI_FLASH=y
+CONFIG_SPI_FLASH=y
+CONFIG_SPI_FLASH_MACRONIX=y
+CONFIG_DM_MMC=y
+CONFIG_MMC_NDS32=y
+CONFIG_FTSDC010=y
+CONFIG_DM_ETH=y
+CONFIG_FTMAC100=y
+CONFIG_BAUDRATE=38400
+CONFIG_DM_SERIAL=y
+CONFIG_SYS_NS16550=y
+CONFIG_DM_SPI=y
+CONFIG_ATCSPI200_SPI=y
+CONFIG_TIMER=y
+CONFIG_ATCPIT100_TIMER=y