hwrng: bcm2835 - Obtain base register via resource

In preparation for consolidating bcm63xx-rng into bcm2835-rng, make sure
that we obtain the base register via platform_get_resource() since we
need to support the non-DT enabled MIPS-based BCM63xx DSL SoCs.

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