Milbeaut Updown counter driver.

1. Overview

This driver runs Updown counter in one of the following modes.
 - Updown count mode
 - Phase difference count mode (2 multiplication)
 - Phase difference count mode (4 multiplication)

The following settings are fixed within the driver. (User can not change)
 - Counter value is reloaded with overflow or underflow
 - Rising edge detection
 - Overflow or underflow interrupt enabled (compare interrupt not support)

2. Usage

 2.1 Basic flow : wait until the counter will be changed to the specified value.
   1) Set user preset reload value
   2) Start Updown counter
   3) Wait for an interrupt in the user program

 2.1 Basic flow : check counter value while running
   1) Set user preset reload value
   2) Start Updown counter
   3) Get current count value while you want to read value
   4) Stop Updown counter

   Each procedures can be done by next command. 
    - Set user preset reload value (sample preset counter value is 9)
      $ echo 9 > /sys/bus/iio/devices/iio\:device0/in_count0_preset

      note: If you don't set in_count0_preset, default value "0xffff" will be set in the driver.
            The timing reflected in the register is at start Updown counter.

    - Start Updown counter
      $ echo 1 > /sys/bus/iio/devices/iio\:device0/in_count0_en

    - Wait for an interrupt in the user program
      $ Please refer to "/tools/iio/iio_event_monitor.c"

    - Get current count value
      $ cat /sys/bus/iio/devices/iio\:device0/in_count0_raw

    - Stop Updown counter
      $ echo 0 > /sys/bus/iio/devices/iio\:device0/in_count0_en

 2.3 option
   Get user preset reload value
   Get Updown counter active status

    - Get user preset reload value
      cat /sys/bus/iio/devices/iio\:device0/in_count0_preset

    - Get Updown counter active status
      cat /sys/bus/iio/devices/iio\:device0/in_count0_en

3. Supplement

The mode is confirmed with the <> value of cms_type on devicetree.
 1: Updown button (updown counter mode)
 2: Rotary encoder (phase difference counter mode)

See Documentation/devicetree/bindings/iio/counter/milbeaut-updown_cnt.txt for details.
