riscv: call pm_power_off from machine_halt / machine_power_off

This way any override of pm_power_off also affects the halt path and
we don't need additional infrastructure for it.

Also remove the pm_power_off export - at least for now we don't have
any modular drivers overriding it.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Reviewed-by: Atish Patra <atish.patra@wdc.com>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>