Refer to main matmul folder for supported data types
and available micro kernels.

Remove reference to micro kernels and winograd transform as
winograd micro kernels are not available now.

Signed-off-by: Felix Thomasmathibalan <felixjohnny.thomasmathibalan@arm.com>

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