mfd: rk808: Make PM function declaration static

Avoids:
  ../drivers/mfd/rk808.c:771:1: warning: symbol 'rk8xx_pm_ops' \
    was not declared. Should it be static?

Fixes: 5752bc43 ("mfd: rk808: Mark pm functions __maybe_unused")
Reviewed-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>