Changes are made to support our FPGA evaluation board, it has DDR memory(0xA0000000-0xB0000000). * Adapt the config.mk to match FPGA evaluation board DDR memory address * Since the RISC-V CPU core frequency of FPGA might change, so we use the fixed TIMER frequency to measure the real CPU core frequency. * And the UART baudrate has to set to 57600bps for Nuclei FPGA evaluation board when CPU core frequency is about 8MHz, otherwise the UART input will not work correctly. Signed-off-by: Huaqi Fang <578567190@qq.com> Reviewed-by: Anup Patel <anup.patel@wdc.com>