mbedtls: Rename Mbed timing implementation

We get a linker warning with the recently added timing module
implementation for Mbed. This is because there is Mbed TLS also ships a
file called timing.c, which we are including in Mbed OS also. With CLI
1, we get an error about unique object files because of the similarly
named implementation files.

    Object file timing.o is not unique! It could be made from: mbed-os/connectivity/mbedtls/source/timing.c mbed-os/connectivity/mbedtls/platform/src/timing.cpp

Rename the Mbed timing module implementation to timing_mbed.cpp to avoid
this naming conflict.

Fixes: b8781e5 ("mbedtls: Add an alt implementation of timing")

Fixes #14759

connectivity/mbedtls/CMakeLists.txt

@@ -16,7 +16,7 @@ target_sources(mbed-mbedtls
  platform/src/mbed_trng.cpp
  platform/src/platform_alt.cpp
  platform/src/shared_rng.cpp
- platform/src/timing.cpp
+ platform/src/timing_mbed.cpp
 
  source/aes.c
  source/aesni.c