riscv: Use do_kernel_power_off()

riscv: Use do_kernel_power_off()

Kernel now supports chained power-off handlers. Use do_kernel_power_off()
that invokes chained power-off handlers. It also invokes legacy
pm_power_off() for now, which will be removed once all drivers will
be converted to the new power-off API.

Acked-by: Palmer Dabbelt <palmer@dabbelt.com>
Signed-off-by: Dmitry Osipenko <digetx@gmail.com>