Add fp16 in/out bf16 Gemm kernel and relevant packing functions

This commit

* Adds bf16 x bf16 = fp16 matmul microkernel with 8x12 output block size
* Lhs/Rhs packing functions that packs and converts the inputs from fp16 to bf16
* Corresponding tests, and modifications to the testing framework, and reference implementation

Signed-off-by: Gunes Bayir <gunes.bayir@arm.com>

Reviewed-by: Viet-Hoa Do <viet-hoa.do@arm.com>
Reviewed-by: Jakub Sujak <jakub.sujak@arm.com>
Reviewed-by: Gunes Bayir <gunes.bayir@arm.com>
Reviewed-by: Emil Ohlsson <emil.ohlsson@arm.com>
Approved-by: Emil Ohlsson <emil.ohlsson@arm.com>