power: qpnp-linear-charger: Add parallel charging support

Extend the linear charger (LBC) driver to support parallel
charging. The LBC acts a secondary charger and its charging
operations are controlled by a primary charger.

In this hardware configuration the BAT_THERM is connected
to the primary charger and the BAT_ID is connected to LBC.

CRs-Fixed: 771178
Change-Id: I22429c6cf5e0bf0178a3221637653a4099c340d3
Signed-off-by: Anirudh Ghayal <aghayal@codeaurora.org>