power: rk818-charger: add TS2 voltage detect when update input current

power: rk818-charger: add TS2 voltage detect when update input current

rk818's input charge voltage limit function doesn't works well. If software
set input current over than charger's max support value, rk818 may cause
charger over current protect which means disconnecting.
To solve this problem, we need to detect vbus voltage by TS2 pin, if vbus
is upper than 4.4v, we can safely adjust input current step by step from
low to high until meeting the target input current value.

Change-Id: I01d63974f251ad8ef0037158b66f4b85d3928baf
Signed-off-by: chenjh <chenjh@rock-chips.com>