iio: adc: Add missing features in PMIC5 ADC

Add the following PMIC5 ADC features:
Update ADC polling time delay for end of conversion (EOC) to match
values on kernel 4.19.
Add missing channels in driver's channel list (bat_therm, bat_id
and some thermistor channels), which may be needed by PMIC5 clients.
Add scaling functions for bat_therm and smb_therm.

Change-Id: I0612560513fbb3f29ff3fda6ad86b939f190410e
Signed-off-by: Jishnu Prakash <jprakash@codeaurora.org>