saradc: refactor the driver based on the iio sub-system

saradc: refactor the driver based on the iio sub-system

PD#146222: saradc: refactor the driver based on the iio sub-system

the sar adc driver has been moved from directory input/saradc
into directory iio/adc, and is refactored base on the standard
iio sub-system.

the iio sub-system provide two ways to access sar adc resources:
1. standard api for consumer driver
it can be found at the file "include/linux/iio/consumer.h".

2. sysfs interface for user space(X: 0-7)
  - obtain 10-bit sample value:
  cat sys/bus/iio/devices/iio:device0/in_voltage[X]_input

  - obtain raw sample value(10-bit or 12-bit):
  cat sys/bus/iio/devices/iio:device0/in_voltage[X]_raw

  - obtain mean raw sample value:
  cat sys/bus/iio/devices/iio:device0/in_voltage[X]_mean_raw

test pass on the gxl skt, axg skt, m200.

Change-Id: I6c8877c782c51a01993557cabc0d43f212a8e524
Signed-off-by: xingyu.chen <xingyu.chen@amlogic.com>