clocksource: mips-gic-timer: Enable counter when CPUs start

In multi-cluster MIPS I6500 systems there is a GIC in each cluster,
each with its own counter. When a cluster powers up the counter will
be stopped, with the COUNTSTOP bit set in the GIC_CONFIG register.

In single cluster systems, it has been fine to clear COUNTSTOP once
in gic_clocksource_of_init() to start the counter. In multi-cluster
systems, this will only have started the counter in the boot cluster,
and any CPUs in other clusters will find their counter stopped which
will break the GIC clock_event_device.

Resolve this by having CPUs clear the COUNTSTOP bit when they come
online, using the existing gic_starting_cpu() CPU hotplug callback. This
will allow CPUs in secondary clusters to ensure that the cluster's GIC
counter is running as expected.

Signed-off-by: Paul Burton <paulburton@kernel.org>
Signed-off-by: Chao-ying Fu <cfu@wavecomp.com>
Signed-off-by: Dragan Mladjenovic <dragan.mladjenovic@syrmia.com>
Signed-off-by: Aleksandar Rikalo <arikalo@gmail.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Tested-by: Serge Semin <fancer.lancer@gmail.com>
Tested-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>