Add a new sensor device API called zio_dev along with supporting
interface and implmentations needed to better encapsulate a large range
of functionality typical sampled input/output (sensor and signal
generator) type devices commonly provide.

Every device has a set of channels and attributes associated with it.
Channels may be attached to a fifo like interface (zio_buf) which
provides a pollable mechanism for obtaining the raw channel data at
a designated watermark level.

Signed-off-by: Tom Burdick <thomas.burdick@gmail.com>

Provide a software only example driver and sample application which
generates a left and right sine wave at CD quality bit width
and rate.

Signed-off-by: Tom Burdick <thomas.burdick@gmail.com>

Rather than providing the attribute values directly in get_attrs
for zio devices and channels, provide descriptions of each attribute.

This should allow a driver to map many attributes directly to registers
or internalized struct members and avoid storing these values in a
zio_variant which is confusing.

Signed-off-by: Tom Burdick <thomas.burdick@gmail.com>