drivers: entropy: gecko: add driver using Secure Element module of EFR32

Some EFR32 SoCs use a secure element subsystem to manage
security features (i.e., TRNG, secure bootloader or cryptographic
functions).

This driver relies on the SE Manager high-level API provided by Silicon
Labs. The API interacts with the SE subsystem, provides helper functions
to achieve cryptographic operations and ensures that only one operation
is running at a time by using mutexes and semaphores.

Instead of relying on the SE Manager from Silicon Labs, one could
recreate the behaviour of the Manager and put the code in the crypto
driver folder and create a dependency for other drivers using the crypto
manager (e.g., keys, entropy).

I went for the SE Manager API as it is already there and supported by
Silicon Labs.

Tested using the random subsystem.

Signed-off-by: Steven Lemaire <steven.lemaire@zii.aero>