Address review comments from feature review

* Rename `matmul` in imatmul interface to `imatmul`
* rename `zero` argument in lhs pack to `pad_ptr`
* Clarify `k_chunk_length` to mean "in bytes"

Signed-off-by: Emil Ohlsson <emil.ohlsson@arm.com>

Approved-by: Jakub Sujak <jakub.sujak@arm.com>