Revert "Arm backend: Remove submodule serialization_lib" (pytorch#13180)

Reverts pytorch#13178

Author: zingo

.gitmodules

@@ -1,6 +1,9 @@
 [submodule "backends/arm/third-party/ethos-u-core-driver"]
 	path = backends/arm/third-party/ethos-u-core-driver
 	url = https://git.gitlab.arm.com/artificial-intelligence/ethos-u/ethos-u-core-driver.git
+[submodule "backends/arm/third-party/serialization_lib"]
+	path = backends/arm/third-party/serialization_lib
+	url = https://git.gitlab.arm.com/tosa/tosa-serialization.git
 [submodule "backends/vulkan/third-party/Vulkan-Headers"]
 	path = backends/vulkan/third-party/Vulkan-Headers
 	url = https://github.com/KhronosGroup/Vulkan-Headers