MLBEDSW-9780: Passthrough all unknown TFLite ops

There can be networks that contains operators that we don't know
about (i.e. not listed in tflite_mapping.cpp). Pass such operators
to CPU.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: I37bd67d440ec6a5d4aafad3d6f17c7f050050cf2