mfd: rk808: Convert RK805 to shutdown/suspend hooks

RK805 has the same kind of dual-role sleep/shutdown pin as RK809/RK817,
so it makes little sense for the driver to have to have two completely
different mechanisms to handle essentially the same thing. Move RK805
over to the shutdown/suspend flow to clean things up.

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