Simplify matmul build files

Original way of organizing matmul files was a bit complicated. A simpler
way would reduce maintenance and simplify on how to add new microkernels

Signed-off-by: Anton Bondarenko <anton.bondarenko@arm.com>

Reviewed-by: Felix Johnny Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>
Approved-by: Jakub Sujak <jakub.sujak@arm.com>