perf docs: Update metric usage for Alibaba's T-Head PMU driver (arm64)

Alibaba's T-Head ali_drw PMU supports DDR bandwidth metrics. Update
its usage in the documentation.
Reviewed-by: John Garry <john.g.garry@oracle.com>
Signed-off-by: Jing Zhang <renyu.zj@linux.alibaba.com>
Acked-by: Ian Rogers <irogers@google.com>
Cc: Namhyung Kim <namhyung@kernel.org>
Cc: Shuai Xue <xueshuai@linux.alibaba.com>
Cc: Will Deacon <will@kernel.org>
Cc: Zhuo Song <zhuo.song@linux.alibaba.com>
Cc: linux-arm-kernel@lists.infradead.org
Cc: linux-doc@vger.kernel.org
Link: https://lore.kernel.org/r/1690528175-2499-4-git-send-email-renyu.zj@linux.alibaba.comSigned-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>

Documentation/admin-guide/perf/alibaba_pmu.rst

@@ -88,6 +88,11 @@ data bandwidth::
     -e ali_drw_27080/hif_rmw/ \
     -e ali_drw_27080/cycle/ -- sleep 10
 
+Example usage of counting all memory read/write bandwidth by metric::
+
+  perf stat -M ddr_read_bandwidth.all -- sleep 10
+  perf stat -M ddr_write_bandwidth.all -- sleep 10
+
 The average DRAM bandwidth can be calculated as follows:
 
 - Read Bandwidth =  perf_hif_rd * DDRC_WIDTH * DDRC_Freq / DDRC_Cycle