iio: adc: Add RRADC driver snapshot

Round robin ADC (RRADC) driver is used on PMI
to read ADC for supported channels such as
battery ID, thermistors.

This snapshot is taken as of msm-4.4
'commit <4f1f60e00d6a> ("Merge "ARM: dts:
msm: add nt35597 sharp panel node for sdm660"")'.

Change-Id: I9d28a58a2f4f1a05d8e99d54d73f33c3542f5a86
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>