dt-bindings: rng: add support for Airoha EN7581 TRNG

Add support for Airoha EN7581 True Random Number generator.

This module can generate up to 4bytes of raw data at times and support
self health test at startup. The module gets noise for randomness from
various source from ADC, AP, dedicated clocks and other devices attached
to the SoC producing true random numbers.

Signed-off-by: Christian Marangi <ansuelsmth@gmail.com>
Reviewed-by: Rob Herring (Arm) <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>