samples: rtio: Simple producer consumer sample

A simple sample showing how an interrupt (or thread) could produce data
a thread (potentially a user mode thread) could then consume.

Some great suggestions added thanks to Luis Ubieda
<luisf@croxel.com> to show multishot in use which avoids extra
syscalls in the tight processing loop of the consumer.

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