iio: adc: exynos-adc: Use proper number of channels for Exynos4x12

[ Upstream commit 103cda6a3b8d2c10d5f8cd7abad118e9db8f4776 ]

Exynos4212 and Exynos4412 have only four ADC channels so using
"samsung,exynos-adc-v1" compatible (for eight channels ADCv1) on them is
wrong.  Add a new compatible for Exynos4x12.

Signed-off-by: Krzysztof Kozlowski <krzk@kernel.org>
Cc: <Stable@vger.kernel.org>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Signed-off-by: Sasha Levin <sashal@kernel.org>