smb138x: support usbin-usbin with external rsense

Currently smb138x only support usbin-usbin parallel architecture
where the battery current from the parallel charger flows through
the batfet of the main charger.

Support configurations where there is remote sensing or an external
rsense where the current from parallel charger directly goes to the
battery.

Change-Id: I5506190d83436bfaea3b6c98b1b311746cf85071
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>