arch: riscv: add common stub reboot function

This patch adds weak sys_arch_reboot() function to avoid build error
with CONFIG_REBOOT=y. Some SoC has already had own reboot function
but others (Ex. qemu boards) faced buld error.

- openisa_rv32m1: Not change
- riscv-ite: Do nothing, remove and use arch/riscv function

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>