power: supply: cpcap-charger: Allow changing constant charge voltage

Let's allow reconfiguring the cpcap-charger max charge voltage and
default to 4.2V that should be safe for the known users.

This allows the users to use 4.35V for the extra capacity if really
needed at a cost of probably shorter battery life. We check the
constant charge voltage limit set by the battery.

Some pieces of the property setting code is based on an earlier patch
from Pavel Machek <pavel@ucw.cz> but limited to configuring the charge
voltage for now.

Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>