docs: pmu: Add Andes PMU node example

Add PMU node example for event index to counter index mapping
and selector value translation of Andes' CPUs.

Currently, there are 4 HPM counters that can be used to monitor
all of the events for each hart.

Signed-off-by: Yu Chien Peter Lin <peterlin@andestech.com>
Reviewed-by: Locus Wei-Han Chen <locus84@andestech.com>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>
Reviewed-by: Anup Patel <anup@brainfault.org>
Reviewed-by: Lad Prabhakar <prabhakar.mahadev-lad.rj@bp.renesas.com>