Add matrix multiplication test

* Add reference implementation for packing LHS, RHS, and the main
  matmul routine.
* Add test for matrix multiplication:
  - Generic test is implemented for all matmul methods.
  - The test checks the arbitrary portion of the output matrix.
  - The test checks the LHS packing, RHS packing and the main
    matmul kernels.

Signed-off-by: Viet-Hoa Do <viet-hoa.do@arm.com>

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