qcom-charger: introduce parallel charging support

Parallel charging increases charging capacity and efficiency by
distributing the current between two charging chips.

PMI8998 feeds the parallel charger via its MID input, and handles
input current limiting in its front-porch FET. As master charger,
PMI8998 is responsible for enabling/disabling the parallel
charger, and the FCC distribution.

To enable parallel charging in software, the following conditions
must be met:
- Strong USBIN input
- Battery present
- In fast or taper charging state
- Attached UFP source
While the enabling/disabling is always under the control of software
the disabling can also be done by hardware in case of fault.

Battery current is usually fixed to the battery rating. The FCC
distribution is simple, a split of 50/50 by default, which can
be changed in runtime.

When taper irq kicks in, the algorithm reduces parallel FCC by 25%.
This puts the charging back in constant current phase until the
next one happens where again the algorithm reduces the FCC by
25%. This continues until the parallel FCC drops to 500mA. At that
time parallel charging is disabled and master continues charging
the rest of constant voltage phase.

CRs-Fixed: 1023703 1030934
Change-Id: Ied7c31d5913df94a288d36ecf06d081d32e07396
Signed-off-by: Harry Yang <harryy@codeaurora.org>
Signed-off-by: Abhijeet Dharmapurikar <adharmap@codeaurora.org>

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