drivers: entropy: Introduce SiWx91x entropy driver

Support for random number generator is required for most of the
cryptographic operations, including support for WiFi and TLS.

This driver has been tested with tests/drivers/entropy:

    *** Booting Zephyr OS build v3.7.0-4339-g1ec5ce05f9f8 ***
    Running TESTSUITE entropy_api
    ===================================================================
    START - test_entropy_get_entropy
    random device is 0x8217298, name is rng@45090000
      0x93
      0x3e
      0xf1
      0x68
      0xd4
      0x22
      0xbf
      0x4d
      0xad
     PASS - test_entropy_get_entropy in 0.012 seconds
    ===================================================================
    TESTSUITE entropy_api succeeded

    ------ TESTSUITE SUMMARY START ------

    SUITE PASS - 100.00% [entropy_api]: pass = 1, fail = 0, skip = 0 ...
     - PASS - [entropy_api.test_entropy_get_entropy] duration = 0.01 ...

    ------ TESTSUITE SUMMARY END ------

    ===================================================================
    RunID: d1547c805699201af769cb01331efcce
    PROJECT EXECUTION SUCCESSFUL

Co-authored-by: Tibor Laczko <tibor.laczko@silabs.com>
Signed-off-by: Tibor Laczko <tibor.laczko@silabs.com>
Signed-off-by: Jérôme Pouiller <jerome.pouiller@silabs.com>