platform: Update Nuclei ux600 platform support

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 <[email protected]>
Reviewed-by: Anup Patel <[email protected]>

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