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>

arch/riscv/kernel/reset.c

@@ -21,6 +21,7 @@ static void default_power_off(void)
 }
 
 void (*pm_power_off)(void) = default_power_off;
+EXPORT_SYMBOL(pm_power_off);
 
 void machine_restart(char *cmd)
 {