MIPS: BCM63XX: Fix missing prototypes

Most of the symbols for which we do not have a prototype can actually be
made static and for the few that cannot, there is already a declaration
in a header for it.

Signed-off-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>

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>