alter adc interface so the chip can be correctly configured.
prepare the driver for i2c implementation
add chan 0 and chan1 to sysfs
remove r3 i2c test
some adc helper functions
implement i2c_read and i2c_write.
use a mutex instead of a spinlock to protect the i2c client (i2c_master_[send/recv] cannot run under a spinlock)