riscv: Move do_reset() to a common place

riscv: Move do_reset() to a common place

We don't have a reset method on any RISC-V board yet. Instead of
adding the same 'unsupported' message for each CPU variant it might
make more sense to add a generic do_reset function for all CPU
variants to lib/, similar to the one for ARM (arch/arm/lib/reset.c).

Suggested-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Signed-off-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>