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: NBoris Brezillon <boris.brezillon@free-electrons.com>
Acked-by: NPeter Korsgaard <peter@korsgaard.com>
Acked-by: NNicolas Ferre <nicolas.ferre@atmel.com>
Signed-off-by: NHerbert Xu <herbert@gondor.apana.org.au>