hwrng: iproc-rng200 - Add support for BCM7278

BCM7278 features a RNG200 hardware random number generator block, add
support for this chip by matching the chip-specific compatible string
and extending the Kconfig dependencies to allow building on ARCH_BRCMSTB
(base platform for 7278).

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