iio: adc: Add ADC5 driver snapshot

ADC5 peripheral is used by client to read VADC
channels. Clients include reading vph_pwr and on board
system thermistors used for thermal mitigation.
Register ADC5 peripheral with IIO framework.

This snapshot is taken as of msm-4.14
'commit <ab8d48dd> ("Merge "diag: Increase the
MHI buffer size to 4k"")'.

Change-Id: I16b1178f7fa44c910fdde533aa199d9928d767e6
Signed-off-by: Siddartha Mohanadoss <smohanad@codeaurora.org>