The regulator for the touchscreen could be:
* A dedicated regulator just for the touchscreen.
* A regulator shared with something else in the system.
* An always-on regulator.

How we want the "reset" line to behave depends a bit on which of those
three cases we're in. Currently the code is written with the
assumption that it has a dedicated regulator, but that's not really
guaranteed to be the case.

The problem we run into is that if we leave the touchscreen powered on
(because someone else is requesting the regulator or it's an always-on
regulator) and we assert reset then we apparently burn an extra 67 mW
of power. That's not great.

Let's instead tie the control of the reset line to the true state of
the regulator as reported by regulator notifiers. If we have an
always-on regulator our notifier will never be called. If we have a
shared regulator then our notifier will be called when the touchscreen
is truly turned on or truly turned off.

Using notifiers like this nicely handles all the cases without
resorting to hacks like pretending that there is no "reset" GPIO if we
have an always-on regulator.

NOTE: if the regulator is on a shared line it's still possible that
things could be a little off. Specifically, this case is not handled
even after this patch:
1. Suspend goodix (send "sleep", goodix stops requesting regulator on)
2. Other regulator user turns off (regulator fully turns off).
3. Goodix driver gets notified and asserts reset.
4. Other regulator user turns on.
5. Goodix driver gets notified and deasserts reset.
6. Nobody resumes goodix.

With that set of steps we'll have reset deasserted but we will have
lost the results of the I2C_HID_PWR_SLEEP from the suspend path. That
means we might be in higher power than we could be even if the goodix
driver thinks things are suspended. Presumably, however, we're still
in better shape than if we were asserting "reset" the whole time. If
somehow the above situation is actually affecting someone and we want
to do better we can deal with it when we have a real use case.
Signed-off-by: Douglas Anderson <dianders@chromium.org>
Signed-off-by: Jiri Kosina <jkosina@suse.cz>