power: msm_charger: control system power

Some chargers can control their current supply for the system. Provide
facility in the msm_charger to choose which charger supplies system
current. It is guaranteed that start charging will always be called
on a charger that has been previously been asked to provide system
current.

Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>