riscv: nx25: lib: Add relative lib funcs to support RISC-V

Add makefile, interrupts.c and boot.c,... functions
to support RISC-V arch.

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>
Cc: Padmarao Begari <Padmarao.Begari@microsemi.com>