raw/skeleton: introduce skeleton rawdev driver

Skeleton rawdevice driver, on the lines of eventdev skeleton, is for
showcasing the rawdev library. This driver implements some of the
operations of the library based on which a test module can be
developed.

Design of skeleton involves a virtual device which is plugged into
VDEV bus on initialization.

Also, enable compilation of rawdev skeleton driver.

Signed-off-by: Shreyansh Jain <shreyansh.jain@nxp.com>