platform: Add Nuclei UX600 platform

* Nuclei UX600 is a 64-bit RISC-V core developed
  by Nuclei System Technology, see https://nucleisys.com/product.php



* The ISA is configurable in hardware on your demand

Signed-off-by: Huaqi Fang <578567190@qq.com>
Reviewed-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>