drivers/bus: Move Arm CCN PMU driver

The arm-ccn driver is purely a perf driver for the CCN PMU, not a bus
driver in the sense of the other residents of drivers/bus/, so let's
move it to the appropriate place for SoC PMU drivers. Not to mention
moving the documentation accordingly as well.

Acked-by: Pawel Moll <pawel.moll@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>