iio: adc: Add SPMI PMIC5 ADC driver

PMIC5 ADC has support for clients to read voltage and
current on supported channels connected to the PMIC.
Clients include reading voltage phone power and on board system
thermistors for thermal management. ADC5 on certain PMIC's has
support to read battery current. Register ADC5 as part of IIO
framework for clients to register and read the single ADC reads.

Change-Id: I4c8f10f2e8e3e69320e4e7f1b8a0da0dca1b8cde
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>