dt-bindings: add device tree binding for Arm CryptoCell trng engine

The Arm CryptoCell is a hardware security engine. This patch adds DT
bindings for its TRNG (True Random Number Generator) engine.

Signed-off-by: Hadar Gat <hadar.gat@arm.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>