Move CPU/GPU files from Core/Runtime to the respective backend folders

Legacy structure contained two libraries core/runtime with two backends
in each.
We reduce the core/runtime libraries to a single library thus merging
the backend files

Signed-off-by: Georgios Pinitas <georgios.pinitas@arm.com>
Change-Id: I69545765fe7a730368105cdbd067d3135ec7a174
Reviewed-on: https://review.mlplatform.org/c/ml/ComputeLibrary/+/6155
Comments-Addressed: Arm Jenkins <bsgcomp@arm.com>
Reviewed-by: Michele Di Giorgio <michele.digiorgio@arm.com>
Tested-by: Arm Jenkins <bsgcomp@arm.com>
diff --git a/docs/ComputeLibrary.dir b/docs/ComputeLibrary.dir
index e08f05e..a57b3cb 100644
--- a/docs/ComputeLibrary.dir
+++ b/docs/ComputeLibrary.dir
@@ -230,7 +230,7 @@
  *  @brief Scalar operations
  */
 
-/** @dir src/core/gpu/cl/kernels/gemm
+/** @dir src/gpu/cl/kernels/gemm
  *  @brief Folder containing all the configuration files for GEMM
  */