arm64/perf: Add Cavium ThunderX PMU support

Support PMU events on Caviums ThunderX SOC. ThunderX supports
some additional counters compared to the default ARMv8 PMUv3:

- branch instructions counter
- stall frontend & backend counters
- L1 dcache load & store counters
- L1 icache counters
- iTLB & dTLB counters
- L1 dcache & icache prefetch counters

Signed-off-by: Jan Glauber <jglauber@cavium.com>
[will: capitalisation]
Signed-off-by: Will Deacon <will.deacon@arm.com>