license: add licenses for exception cases

license: add licenses for exception cases

The license/exceptions.txt file lists a small number of files
that have licenses that are exceptions to the three main
licenses defined in the Intellectual Property Policy of the
DPDK Charter.

The three exception licenses are MIT, ISC and BSD-2-Clause.
Each of these licenses states that the content of the license
message should be included in the code distribution. This
change adds the text of the MIT, ISC and BSD-2-Clause licenses
as defined on the SPDX website.

Cc: stable@dpdk.org
Signed-off-by: John McNamara <john.mcnamara@intel.com>
Acked-by: Bruce Richardson <bruce.richardson@intel.com>