iio: adc: exynos_adc: Add support for exynos7

The ADC on exynos7 is quite similar to ADCv2. The differences are as
follows:
	- exynos7-adc has 8 input channels (as against 10 in ADCv2).
	- exynos7 does not include an ADC PHY control register.
	- Some ADC_CON2 register bits being used in ADCv2 are listed as
	  reserved in exynos7-adc. This results in a different init_hw
	  function for exynos7.

Signed-off-by: Abhilash Kesavan <a.kesavan@samsung.com>
Reviewed-by: Chanwoo Choi <cw00.choi@samsung.com>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>