MIPS: Cobalt: Fix missing prototypes

Fix missing prototypes warnings for cobalt_machine_halt() and
cobalt_machine_restart() by moving their prototypes to cobalt.h which is
included by setup.c.

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