clocksource: mips-gic-timer: Mark GIC timer as unstable if ref clock changes

Currently clocksource framework doesn't support the clocks with variable
frequency. Since MIPS GIC timer ticks rate might be unstable on some
platforms, we must make sure that it justifies the clocksource
requirements. MIPS GIC timer is incremented with the CPU cluster reference
clocks rate. So in case if CPU frequency changes, the MIPS GIC tick rate
changes synchronously. Due to this the clocksource subsystem can't rely on
the timer to measure system clocks anymore. This commit marks the MIPS GIC
based clocksource as unstable if reference clock (normally it's a CPU
reference clocks) rate changes. The clocksource will execute a watchdog
thread, which lowers the MIPS GIC timer rating to zero and fallbacks to a
new stable one.

Note we don't need to set the CLOCK_SOURCE_MUST_VERIFY flag to the MIPS
GIC clocksource since normally the timer is stable. The only reason why
it gets unstable is due to the ref clock rate change, which event we
detect here in the driver by means of the clocks event notifier.

Signed-off-by: Serge Semin <[email protected]>
Cc: Alexey Malahov <[email protected]>
Cc: Thomas Bogendoerfer <[email protected]>
Cc: Paul Burton <[email protected]>
Cc: Ralf Baechle <[email protected]>
Cc: Alessandro Zummo <[email protected]>
Cc: Alexandre Belloni <[email protected]>
Cc: Arnd Bergmann <[email protected]>
Cc: Rob Herring <[email protected]>
Cc: [email protected]
Cc: [email protected]
Cc: [email protected]
Signed-off-by: Daniel Lezcano <[email protected]>
Link: https://lore.kernel.org/r/[email protected]

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