rtio: Add macro to define iodev submission queue

The iodev submission queue existed but wasn't easily constructed as
there was no macro to define it.

Adds a simple macro wrapper around RTIO_SPSC_DEFINE for creating
the RTIO IO Device Submission Queue enabling each IO device to
have its own pending queue of requests.

The sample has been updated to use the iodev submission queue rather
than a k_msgq.

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