drivers: iio: adc: Add support for suspend to disk

Add support to adc driver for suspend to disk feature.
Free interrupts during hibernation and re-register them during
resume.

Change-Id: I71d2aed51f0697e712db59dcc1ef4af0fa98aae4
Signed-off-by: Rama Krishna Phani A <rphani@codeaurora.org>