Signed-off-by: Charley Chu <haoc@cypress.com>

Use a singleton Mutex in platforms_alt functions, to be shared
with the trng function, to save RAM. Rename `platform_alt.c`
to `platform_alt.cpp` as the mutex is in a `singletonPtr`
template class.

Like all HAL APIs, the calls in trng_api.h are not expected to
be thread-safe.

All current accesses to the TRNG HAL are currently via
`mbedtls_hardware_poll`.  Mbed TLS does not currently serialise these
calls itself, as `MBEDTLS_THREADING_C` is not enabled. But even if
Mbed TLS's own accesses were serialised, there are other direct
users of `mbedtls_hardware_poll` such as randLIB, that need to use
direct calls due to lack of API to extract entropy from Mbed TLS.

As such it makes sense to treat `mbedtls_hardware_poll` as a de facto
public Mbed OS API, akin to the C++ veneers on top of the HAL, and add a
PlatformMutex there so that it is safe for multithreaded use.