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>

Documentation/perf/thunderx2-pmu.txt

+Cavium ThunderX2 SoC Performance Monitoring Unit (PMU UNCORE)
+=============================================================
+
+The ThunderX2 SoC PMU consists of independent, system-wide, per-socket
+PMUs such as the Level 3 Cache (L3C) and DDR4 Memory Controller (DMC).
+
+The DMC has 8 interleaved channels and the L3C has 16 interleaved tiles.
+Events are counted for the default channel (i.e. channel 0) and prorated
+to the total number of channels/tiles.
+
+The DMC and L3C support up to 4 counters. Counters are independently
+programmable and can be started and stopped individually. Each counter
+can be set to a different event. Counters are 32-bit and do not support
+an overflow interrupt; they are read every 2 seconds.
+
+PMU UNCORE (perf) driver:
+
+The thunderx2_pmu driver registers per-socket perf PMUs for the DMC and
+L3C devices.  Each PMU can be used to count up to 4 events
+simultaneously. The PMUs provide a description of their available events
+and configuration options under sysfs, see
+/sys/devices/uncore_<l3c_S/dmc_S/>; S is the socket id.
+
+The driver does not support sampling, therefore "perf record" will not
+work. Per-task perf sessions are also not supported.
+
+Examples:
+
+# perf stat -a -e uncore_dmc_0/cnt_cycles/ sleep 1
+
+# perf stat -a -e \
+uncore_dmc_0/cnt_cycles/,\
+uncore_dmc_0/data_transfers/,\
+uncore_dmc_0/read_txns/,\
+uncore_dmc_0/write_txns/ sleep 1
+
+# perf stat -a -e \
+uncore_l3c_0/read_request/,\
+uncore_l3c_0/read_hit/,\
+uncore_l3c_0/inv_request/,\
+uncore_l3c_0/inv_hit/ sleep 1