MIPS: bmips: Fix compiler warning observed on W=1 build

The function arch_sync_dma_for_cpu_all() was used but was
missing a prototype declaration.

Signed-off-by: Jim Quinlan <jim2101024@gmail.com>
Reported-by: kernel test robot <lkp@intel.com>
Acked-by: Florian Fainelli <f.fainelli@gmail.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>