power: regulator: rk8xx: Allow input current/charger shutdown configuration

The RK818 PMIC contains a charger. Add very basic charger functionality
to be able to regulate the USB input current and charger shutdown limits.

Signed-off-by: Wadim Egorov <w.egorov@phytec.de>
Reviewed-by: Simon Glass <sjg@chromium.org>
Acked-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>
Reviewed-by: Philipp Tomsich <philipp.tomsich@theobroma-systems.com>