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 <[e-mail]>
Signed-off-by: Jing Zhang <[e-mail]>
Acked-by: Ian Rogers <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Cc: Shuai Xue <[e-mail]>
Cc: Will Deacon <[e-mail]>
Cc: Zhuo Song <[e-mail]>
Cc: [e-mail]
Cc: [e-mail]
Link: https://lore.kernel.org/r/[e-mail]
Signed-off-by: Arnaldo Carvalho de Melo <[e-mail]>

Documentation/admin-guide/perf/alibaba_pmu.rst

···
     -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