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 <9b3e8a81dcee1d1b92db6429f9abd83cc22c01f4>
(Merge "qcom: smb2: ensure QC adapter is at 5V at shutdown")

Change-Id: Id110ac95e264951be9cb49e1a97bf5f545e5e9cb
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>