Add a DT node for the TRNG (True Random Number Generator) block.

Keep this block enabled as it does not depend on any external connection,
and thus should be available on all boards.
Signed-off-by: Boris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: Nicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>