hwmon: qpnp-adc-voltage: Support CHG_TEMP channel on PMI8994

Add support to toggle SMBCHG_OTG_LOW_PWR_OPTIONS[4] for PMI8994
before and after performing a VADC conversion for CHG_TEMP channel.
This is needed to enable the charger circuitry before reading the
CHG_TEMP channel for a valid temperature reading.

Change-Id: Ia08fa6a7af6c48481305bcd453df625048a4c5eb
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>