mfd: rk808: Stop using syscore ops

Setting the SLEEP pin to its shutdown function for appropriate PMICs
doesn't need to happen in single-CPU context, so there's really no point
involving the syscore machinery. Hook it up to the standard driver model
shutdown method instead. This also obviates the issue that the syscore
ops weren't being unregistered on probe failure or module removal.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>