arch: unicore32: ksyms: export 'pm_power_off' to avoid compiling failure.

Two driver modules need 'pm_power_off', so export it.

The related error (with allmodconfig under unicore32):

MODPOST 4039 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]>
Acked-by: Xuetao Guan <[e-mail]>
Signed-off-by: Xuetao Guan <[e-mail]>

arch/unicore32/kernel/process.c

···
  * Function pointers to optional machine specific functions
  */
 void (*pm_power_off)(void) = NULL;
+EXPORT_SYMBOL(pm_power_off);
 
 void machine_power_off(void)
 {