drivers: Introduce AIODEV subsystem ...

AIODEV/Aiodevice is a analog I/O framework that can be thought of as a
simplified hybrid between 'hwmon' and 'IIO' subsystems of Linux kernel

This commit is very heavily based on 'iodevice' framework proposal
written by Sascha Hauer.

Signed-off-by: Andrey Smirnov <andrew.smirnov@gmail.com>
Signed-off-by: Sascha Hauer <s.hauer@pengutronix.de>