perf/arm-cci: Clean up model discovery

Since I am the self-appointed of_device_get_match_data() police, it's
only right that I should clean up this driver while I'm otherwise
touching it. This also reveals that we're passing around a struct
platform_device in places where we only ever care about its regular
device, so straighten that out in the process.

Acked-by: Punit Agrawal <punit.agrawal@arm.com>
Acked-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>