clocksource/drivers/mips-gic-timer: Always use cluster 0 counter as clocksource

In a multi-cluster MIPS system, there are multiple GICs - one in each
cluster - each of which has its independent counter. The counters in
each GIC are not synchronized in any way, so they can drift relative
to one another through the lifetime of the system. This is problematic
for a clock source which ought to be global.

Avoid problems by always accessing cluster 0's counter, using
cross-cluster register access. This adds overhead so it is applied only
on multi-cluster systems.

Signed-off-by: Paul Burton <[email protected]>
Signed-off-by: Chao-ying Fu <[email protected]>
Signed-off-by: Dragan Mladjenovic <[email protected]>
Signed-off-by: Aleksandar Rikalo <[email protected]>
Tested-by: Serge Semin <[email protected]>
Acked-by: Thomas Bogendoerfer <[email protected]>
Tested-by: Gregory CLEMENT <[email protected]>
Link: https://lore.kernel.org/r/[email protected]
Signed-off-by: Daniel Lezcano <[email protected]>

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