Add Ethos-U operator delegate for TFLite

A delegate has been added to support running the Ethos-U operator in
networks on the NPU. The delegate is built as a shared library and can
be loaded as an external delegate for TFLite.

Change-Id: I391f9bfb7fabe3ea871b2e4c862b17095e0e8e41
Signed-off-by: Mikael Olsson <mikael.olsson@arm.com>