iio: adc: exynos_adc: Control special clock for ADC

iio: adc: exynos_adc: Control special clock for ADC

This patch control special clock for ADC in Exynos series's FSYS block.
If special clock of ADC is registerd on clock list of common clk framework,
Exynos ADC drvier have to control this clock.

- 'adc' clock: bus clock for ADC
- 'sclk_tsadc' clock: special clock for ADC which provide clock to internal ADC

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>