MIPS: csrc-r4k: Select HAVE_UNSTABLE_SCHED_CLOCK if SMP && 64BIT

csrc-r4k suffers from SMP synchronization overhead.

Select HAVE_UNSTABLE_SCHED_CLOCK to workaround drift
between the CPUs on the system. HAVE_UNSTABLE_SCHED_CLOCK
requires cmpxchg64, so enable it for 64 bits only.

Signed-off-by: Jiaxun Yang <jiaxun.yang@flygoat.com>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>