smb138x-charger: enable parallel current sensing

When parallel charging is enabled the charge current read by the fuel
gauage will not include the parallel charging current. Enable VCHG to
report the parallel charge current.

Change-Id: I6090d4163e760dc02a7dce45f1246bb59b2e195b
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>