MLBEDSW-7545: Fix assert when serializing a tensor

- The assert triggers when a constant tensor is being assigned
to buffer 0 and that is a violation.
- The test case that triggered this problem revealed an error in
the reader code. If the input tensor has constant data it should
be using a Const op. Before this fix it was assigned a
Placeholder op and the tensor ended up in the scratch area
instead of the permanent area.

Change-Id: I4f92fb5ec1f0dc594defbaca0335eabe68fd5137
Signed-off-by: Johan Alfven <johan.alfven@arm.com>