riscv: export pm_power_off again

Commit bf0102a0 ("riscv: call pm_power_off from machine_halt /
machine_power_off") removed the export of pm_power_off, but it is used by
several modules:

    ERROR: "pm_power_off" [drivers/mfd/rk808.ko] undefined!
    ERROR: "pm_power_off" [drivers/mfd/max8907.ko] undefined!
    ERROR: "pm_power_off" [drivers/mfd/axp20x.ko] undefined!
    ERROR: "pm_power_off" [drivers/char/ipmi/ipmi_poweroff.ko] undefined!

Signed-off-by: Andreas Schwab <schwab@suse.de>
Fixes: bf0102a0 ("riscv: call pm_power_off from machine_halt / machine_power_off")
Signed-off-by: Paul Walmsley <paul.walmsley@sifive.com>