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: NChen Gang <gang.chen.5i5j@gmail.com>
Acked-by: NXuetao Guan <gxt@mprc.pku.edu.cn>
Signed-off-by: NXuetao Guan <gxt@mprc.pku.edu.cn>

arch/unicore32/kernel/process.c

@@ -60,6 +60,7 @@ void machine_halt(void)
  * Function pointers to optional machine specific functions
  */
 void (*pm_power_off)(void) = NULL;
+EXPORT_SYMBOL(pm_power_off);
 
 void machine_power_off(void)
 {