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