power: smb138x-charger: Add parallel charger logic

SMB138x charger can be configured as a slave in parallel charger
configuration. Register parallel power_supply for it and implement
relevant interfaces for main charger to control it, including
allot ICL/FCC, enable/disable input, etc.

CRs-Fixed: 1032013
Change-Id: If867d4ff865296fff9d022547c088bc94453367d
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>
Signed-off-by: Fenglin Wu <fenglinw@codeaurora.org>