power: supply: cpcap-battery: Keep track of state for capacity reporting

Save the battery empty and full states so we can use those to estimate
the battery capacity in the following patches.

If the user provides us with charge_full value (which it could save in a
permanent storage between reboots), initialize low and high counter_uah
with calculated values.

If we hit battery low once, we should stick on reporting it until the
charger is connected. This way low->counter_uah will be updated
properly, and that will allow us to get more accurate charge_full value.

Based on an earlier patch by Tony Lindgren with charge_full usage and
other improvments done by Arthur Demchenkov.

Cc: Arthur Demchenkov <spinal.by@gmail.com>
Cc: Carl Philipp Klemm <philipp@uvos.xyz>
Cc: Merlijn Wajer <merlijn@wizzup.org>
Cc: Pavel Machek <pavel@ucw.cz>
Co-developed-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Arthur Demchenkov <spinal.by@gmail.com>
[tony@atomide.com: combined earlier patches, updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>