mfd: rk808: Prepare rk805 for poweroff

RK805 has SLEEP signal, which can put the device into SLEEP or OFF
mode. The default is SLEEP mode.

However, when the kernel performs power-off (actually the ATF) the
device will not go fully off and this will result in higher power
consumption and inability to wake the device with RTC alarm.

The solution is to enable pm_power_off_prepare function, which will
configure SLEEP pin for OFF function.

Signed-off-by: Stefan Mavrodiev <stefan@olimex.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>