arc: export symbol for pm_power_off in reset.c

Need export symbol for it, or can not pass compiling, the related error
with allmodconfig:

MODPOST 2994 modules
ERROR: "pm_power_off" [drivers/mfd/retu-mfd.ko] undefined!
ERROR: "pm_power_off" [drivers/char/ipmi/ipmi_poweroff.ko] undefined!

Signed-off-by: Chen Gang <[e-mail]>
Signed-off-by: Vineet Gupta <[e-mail]>

arch/arc/kernel/reset.c

···
 }
 
 void (*pm_power_off) (void) = NULL;
+EXPORT_SYMBOL(pm_power_off);