MIPS: SGI-IP28: disable use of ll/sc in kernel

SGI-IP28 systems only use broken R10k rev 2.5 CPUs, which could lock
up, if ll/sc sequences are issued in certain order. Since those systems
are all non-SMP, we can disable ll/sc usage in kernel.

Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>