[mlir][GPU] Update CODEOWNERS (#155258)

Updating the CODEOWNERS file to more easily perform the MLIR GPU dialect
maintainer role.

Author: fabianmcg

.github/CODEOWNERS

@@ -112,6 +112,9 @@
 /mlir/**/NVGPU*/ @grypp
 /mlir/test/**/CUDA/ @grypp
 
+# MLIR GPU Dialect
+/mlir/**/GPU*/ @fabianmcg
+
 # MLIR NVVM Dialect in MLIR
 /mlir/**/LLVMIR/**/BasicPtxBuilderInterface* @grypp
 /mlir/**/NVVM* @grypp