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>