arch: hexagon: kernel: reset.c: use function pointer instead of function for pm_power_off and export it

'pm_power_off' is a function pointer, not a function, so need change
its type, and also need export it, or can not pass compiling with
allmodconfig. The related error:

    MODPOST 2879 modules
  ERROR: "pm_power_off" [drivers/char/ipmi/ipmi_poweroff.ko] undefined!
Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Kuo <rkuo@codeaurora.org>