To avoid collisions between the psa `crypto_platform.h` file and the
platform specific `crypto_platform.h` file, for the init \ terminate
functions, rename the latter to `crypto_device_platform`.

Rename `obsolete_ctx` to `unused_ctx` as it is simply unused.

1. Use atomic operations to increase and decrease counter.
2. Style fixes.
Remove unused function declaration.

Make the platform context a global variable,
adding the refernce counter to it.

1. Move the `mbedtls_platform_context` to be platform code, in `features/mbedtls/platfrom/`.
2. Add static refernce counter, to setup and teardown the platform code only once.
3. Adjust Cryptocell porting accordingly.