hwrng: airoha - add support for Airoha EN7581 TRNG

Add support for Airoha TRNG. The Airoha SoC provide a True RNG module
that can output 4 bytes of raw data at times.

The module makes use of various noise source to provide True Random
Number Generation.

On probe the module is reset to operate Health Test and verify correct
execution of it.

The module can also provide DRBG function but the execution mode is
mutually exclusive, running as TRNG doesn't permit to also run it as
DRBG.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Martin Kaiser <martin@kaiser.cx>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>