iio: adc: cpcap: Fix default register values and battery temperature

Looking at the register dumps from Android kernel on droid 4, I noticed
the values with the mainline kernel don't match. Let's fix this by
initializing the ADC registers to what Android does.

For getting correct values from the battery thermistor, we need to
toggle the CPCAP_BIT_THERMBIAS_EN when measuring battery temperature
to get correct battery temperatures. And looks like we also need to
wait a little bit before reading the battery temperature as otherwise
the results are inaccurate.

Cc: Marcel Partap <mpartap@gmx.net>
Cc: Michael Scott <michael.scott@linaro.org>
Cc: Sebastian Reichel <sre@kernel.org>
Signed-off-by: Tony Lindgren <tony@atomide.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>