devlink: add documentation for ionic device driver

The IONIC device driver allocates a devlink and reports versions. Add
documentation for this driver.

Signed-off-by: Jacob Keller <jacob.e.keller@intel.com>
Cc: Shannon Nelson <snelson@pensando.io>
Acked-by: Shannon Nelson <snelson@pensandi.io>
Signed-off-by: David S. Miller <davem@davemloft.net>