hwrng: bcm63xx - fix non device tree compatibility

Allow building when OF is not enabled as suggested by Florian

Signed-off-by: Álvaro Fernández Rojas <noltari@gmail.com>
Reported-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Herbert Xu <herbert@gondor.apana.org.au>