hwrng: atmel - add DT support

Add DT support.

Make the driver depend on CONFIG_OF as at91sam9g45 was the only SoC making
use of the TRNG block and this SoC is now fully migrated to DT.

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>