iio: adc: exynos_adc: Add support for s3c24xx ADC

This patch add support for s3c2410/s3c2416/s3c2440/s3c2443 ADC. The s3c24xx
is alomost same as ADCv1. But, There are a little difference as following:
- ADCMUX register address
- ADCDAT mask (10 bit or 12 bit ADC resolution according to SoC version)
- s3c24xx/s3c64xx has not included ADC_PHY enable register

Signed-off-by: Chanwoo Choi <cw00.choi@samsung.com>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Jonathan Cameron <jic23@kernel.org>