MLBEDSW-10886: Don't put constant CPU-only tensors in ReadOnly

Constant tensors that are not read or written from the NPU don't
need to be in the ReadOnly memory area. They can be stored as
TFLite buffers only.

This matches the Vela/Python behavior.

This patch relates to commit 2b8c8725 and d4c6dbd7.

Signed-off-by: Johan Gunnarsson <johan.gunnarsson@arm.com>
Change-Id: I414a7eaf22727cfa1a3e71318413c6fe6b584bac