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>

arch/mips/Kconfig

@@ -1084,6 +1084,7 @@ config CSRC_IOASIC
 
 config CSRC_R4K
 	select CLOCKSOURCE_WATCHDOG if CPU_FREQ
+	select HAVE_UNSTABLE_SCHED_CLOCK if SMP && 64BIT
 	bool
 
 config CSRC_SB1250