hwrng: atmel - Add TRNG DT binding doc

Document DT bindings of Atmel's TRNG (True Random Number Generator) IP.

Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Peter Korsgaard <peter@korsgaard.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>