soc: samsung: exynos-pmu: enable CPU hotplug support for gs101

Some additional register writes are required when hotplugging CPUs
on gs101, without these the system hangs when hotplugging.

Specifically a CPU_INFORM register needs to be programmed with
a hint value which is used by the EL3 firmware (el3mon) and the
pmu-intr-gen registers need to be programmed.

With this patch applied, and corresponding DT update CPU hotplug
now works as expected. e.g.

echo 0 > /sys/devices/system/cpu/cpu6/online
echo 1 > /sys/devices/system/cpu/cpu6/online

Note: to maintain compatibility with older DTs that didn't specify
pmu-intr-gen phandle only a warning is issued if the syscon can't
be obtained.

Signed-off-by: Peter Griffin <peter.griffin@linaro.org>
Link: https://lore.kernel.org/r/20250506-contrib-pg-cpu-hotplug-suspend2ram-fixes-v1-v4-5-9f64a2657316@linaro.org


[krzk: few blank line and white-space alignment fixes from checkpatch]
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>