qpnp-smb2: expose parallel slave percentage via battery psy

Expose the parallel distribution percentage and allow it to be
modified.

Change-Id: I0c9221c6a748aff5e8fbe889316f729155eded3e
Signed-off-by: Nicholas Troast <ntroast@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>