power: qpnp-charger: add support for BTC devicetree configuration

Add support to specify the BTC threshold configuration percentage
values via devicetree. These percentage value reflect
the resulting voltage on the BAT_THERM rail pulled up by a
1.8 V reference voltage VREF_BAT_THM.

Support to disable the BTC thresholds completely has also been
added.

The available configurations are:
  * BAT_TEMP_HOT_THD:  25%, 35%
  * BAT_TEMP_COLD_THD:  70%, 80%

The following device tree properties have been added:
  * qcom,batt-hot-percentage
  * qcom,batt-cold-percentage
  * qcom,btc-disabled

CRs-Fixed: 497704
Change-Id: I864e050d3c0cb751f6e2876a21525a8370384f9f
Signed-off-by: David Keitel <dkeitel@codeaurora.org>