Use GitHub mirrors of mlplatform submodules to unbreak ExecuTorch CI

CI jobs have been failing due to availability issues with these submodules. mirrors should unbreak CI.

Internal: S470174

Differential Revision: [D66255556](https://our.internmc.facebook.com/intern/diff/D66255556/)

ghstack-source-id: 254626070
Pull Request resolved: #6989

Author: swolchok

.gitmodules

@@ -1,9 +1,9 @@
 [submodule "backends/arm/third-party/ethos-u-core-driver"]
 	path = backends/arm/third-party/ethos-u-core-driver
-	url = https://git.mlplatform.org/ml/ethos-u/ethos-u-core-driver.git/
+	url = https://github.com/pytorch-labs/ethos-u-core-driver-mirror
 [submodule "backends/arm/third-party/serialization_lib"]
 	path = backends/arm/third-party/serialization_lib
-	url = https://git.mlplatform.org/tosa/serialization_lib.git/
+	url = https://github.com/pytorch-labs/tosa_serialization_lib-mirror
 [submodule "backends/vulkan/third-party/Vulkan-Headers"]
 	path = backends/vulkan/third-party/Vulkan-Headers
 	url = https://github.com/KhronosGroup/Vulkan-Headers