Documentation: perf: Add documentation for ThunderX2 PMU uncore driver

The SoC has PMU support in its L3 cache controller (L3C) and in the
DDR4 Memory Controller (DMC).

Signed-off-by: Ganapatrao Kulkarni <ganapatrao.kulkarni@cavium.com>
[will: minor spelling and format fixes, dropped events list]
Signed-off-by: Will Deacon <will.deacon@arm.com>