compute_square_kernel.cl: add CL file used at xe_compute.c

Provide the cl file that it was used to produce the OpenCL
Kernel used by xe_compute, and document how the binary at
xe_compute.h was produced.

Reviewed-by: Zbigniew Kempczyński <zbigniew.kempczynski@intel.com>
Signed-off-by: Mauro Carvalho Chehab <mchehab@kernel.org>