hwrng: bcm63xx - fix device tree compilation

Adds missing include that resulted in implicit device tree functions errors.

Fixes: 7b651706

 ("hwrng: bcm63xx - add device tree support")
Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>