smb-lib: smb138x-charger: add parallel current to parallel psy

The measured battery current can be read from the parallel slave
charger. Expose this through the CURRENT_NOW property in the parallel
power supply.

Change-Id: Icd717147adc018a076a72bd5ce1a52a765f34f7b
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>