MLBEDSW-7408: MLCE: Crash when serialising model LSTM

 - Added checking and reporting of missing operator attributes when
reading and writing TFLite file
 - Added a TFLite semantic check to ensure that all required attribute
fields of builtin operators are read
 - Added some sanity checks for RESHAPE operators that run on the
Ethos-U
 - Stopped CPU operators from having their attributes modified

Change-Id: I05700681acdb09554f5945819717c08a9457295c
Signed-off-by: Tim Hall <tim.hall@arm.com>