ARM: SAMSUNG: ADC Channel selection

In S5PV210/S5PC110/EXYNOS4, ADCMUX channel selection uses ADCMUX
register, not ADCCON register. This patch corrects the behavior of
SAMSUNG-ADC for such CPUs.

Signed-off-by: MyungJoo Ham <myungjoo.ham@samsung.com>
Signed-off-by: Kyungmin Park <kyungmin.park@samsung.com>
Signed-off-by: Kukjin Kim <kgene.kim@samsung.com>