power: supply: cpcap-charger: Limit voltage to 4.2V for battery

There have been some cases of droid4 battery bulging that seem to be
related to being left connected to the charger for several weeks.

It is suspected that the 4.35V charge voltage configured for the battery
is too much in the long run, so lets limit the charge voltage to 4.2V.
It could also be that the batteries are just getting old.

We don't really want to just change the charge voltage to 4.2V as Android
may have charged the battery to 4.35V as pointed out by Pavel Machek.

To add checks for battery voltage, the driver needs to understand the
voltage it's charging at, and also needs to better understand it's
charger state. Right now it only understands connect and disconnect,
while now we need to know also a connected state but not charging.

So let's add better charger state handling with help of chrgcurr2 interrupt
for detecting charge full and retry, and add a check for battery voltage
before we start charging. And then we finally can lower the charge voltage
to 4.2V.

Note that we've been using the same register values as the Android distros
on droid4, so it is suspected that the same problem also exists in Android.

Cc: Pavel Machek <pavel@ucw.cz>
Cc: Rob Herring <robh+dt@kernel.org>
Reported-by: Merlijn Wajer <merlijn@wizzup.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>