power: supply: bq25890: Add support for setting user charge current and voltage limit

Let user set battery charge current and voltage limit via sysfs. This is
useful in case the user space needs to reduce charge current to keep the
system within thermal limits. The maximum charge current and voltage are
still limited to "ti,charge-current" and "ti,battery-regulation-voltage"
values to avoid damaging the hardware in case too high values are set by
user space.

Reviewed-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Marek Vasut <marex@denx.de>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>