riscv: nx25: include: Add header files to support RISC-V

Add header files for RISC-V.
Cache, ptregs, data type and other definitions are included.

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>