drivers/bus: arm-cci: fix build warnings

When the arm-cci driver is enabled, but both CONFIG_ARM_CCI5xx_PMU and
CONFIG_ARM_CCI400_PMU are not, we get a warning about how parts of
the driver are never used:

drivers/perf/arm-cci.c:1454:29: error: 'cci_pmu_models' defined but not used [-Werror=unused-variable]
drivers/perf/arm-cci.c:693:16: error: 'cci_pmu_event_show' defined but not used [-Werror=unused-function]
drivers/perf/arm-cci.c:685:16: error: 'cci_pmu_format_show' defined but not used [-Werror=unused-function]

Marking all three functions as __maybe_unused avoids the warnings in
randconfig builds. I'm doing this lacking any ideas for a better fix.

Fixes: 3de6be7a ("drivers/bus: Split Arm CCI driver")
Reviewed-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Will Deacon <will.deacon@arm.com>