power: supply: cpcap-charger: Fix flakey reboot with charger connected

If we have a USB charger connected, reboot is flakey and often fails to
reboot the device with the charger LED staying on.

Let's fix this by implementing .shutdown.

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: Tony Lindgren <tony@atomide.com>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>