[MIPS] SB1250: Fix bugs/warnings by creative use of volatile.

arch/mips/mm/pg-sb1.c

arch/mips/pci/pci-bcm1480.c

arch/mips/pci/pci-sb1250.c

arch/mips/sibyte/bcm1480/smp.c

arch/mips/sibyte/swarm/setup.c

drivers/net/sb1250-mac.c

include/asm-mips/sibyte/sb1250.h