power: qpnp-smbcharger: smb135x-charger: improve parallel charging

The current implementation of parallel charging has a few issues.

	1. During constant voltage charging of the parallel charger,
	   there can be buck fights that cause noise on vbat, which is
	   undesirable.
	2. Doing parallel charging only in constant current charging is
	   inefficient because a large portion of the charge cycle is
	   being spent during constant voltage charging now.

Fix these issues and improve the parallel charging algorithm by taking a
different approach. The new algorithm sets the float voltage of the
parallel charger to 4.45V, so it will always act as a current source.
When the primary charger reaches constant voltage charging, the taper
charge interrupt will notify the software and the software will reduce
the fast charge current of the parallel charger by one step. This
repeats until the parallel charger turns off.

This new algorithm will help achieve a faster and less thermal intensive
charge cycle, as well as being better for the battery.

CRs-Fixed: 713189
Change-Id: I8aba05f6648e1085c346246b0ff1a89352185fbd
Signed-off-by: Xiaozhe Shi <xiaozhes@codeaurora.org>