power: supply: cpcap-battery: Add charge_full property

Add charge_full property and let user update it. This is needed for
capacity reporting in the following patches.

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>
Signed-off-by: Arthur Demchenkov <spinal.by@gmail.com>
[tony@atomide.com: updated comments]
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>