Add IMATMUL version of QAI8

This change adds a IMATMUL version of the QAI8 kernel, as well as
changes unit tests to call into this new kernel and adds an interface
for this kernel as well

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

Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Jakub Sujak <jakub.sujak@arm.com>