MIPS: BCM63XX: add RNG driver platform_device stub

Signed-off-by: Florian Fainelli <florian@openwrt.org>
Cc: linux-mips@linux-mips.org
Cc: mpm@selenic.com
Cc: herbert@gondor.apana.org.au
Patchwork: https://patchwork.linux-mips.org/patch/3325/


Signed-off-by: Ralf Baechle <ralf@linux-mips.org>