expose three adc channels to userspace

adc0 can be accessed by "cat /sys/devices/platform/rk30-adc/adc0"
adc0 adc1 adc2 are async read
adc_sync_0 adc_sync_1 adc_sync_2 are sync read
Merge pull request #2 from clcarwin/patch-1

expose three adc channels to userspace