mfd: db8500-prcmu: Use ANSI function declaration

Silences the following warning:
drivers/mfd/db8500-prcmu.c:2322:25: warning:
non-ANSI function declaration of function 'prcmu_ac_sleep_req'

Acked-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Sachin Kamat <sachin.kamat@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>