docs: perf: Add imx-ddr to documentation index

devices/imx8_ddr0/format/. The "events" directory describes the events types

hardware supported that can be used with perf tool, see /sys/bus/event_source/

devices/imx8_ddr0/events/.

  e.g.::

    .. code-block:: bash

        perf stat -a -e imx8_ddr0/cycles/ cmd

        perf stat -a -e imx8_ddr0/read/,imx8_ddr0/write/ cmd

  Filter is defined with two configuration parts:

  --AXI_ID defines AxID matching value.

  --AXI_MASKING defines which bits of AxID are meaningful for the matching.

        0：corresponding bit is masked.

        1: corresponding bit is not masked, i.e. used to do the matching.

      - 0: corresponding bit is masked.

      - 1: corresponding bit is not masked, i.e. used to do the matching.

  AXI_ID and AXI_MASKING are mapped on DPCR1 register in performance counter.

  When non-masked bits are matching corresponding AXI_ID bits then counter is

  This filter doesn't support filter different AXI ID for axid-read and axid-write

  event at the same time as this filter is shared between counters.

  e.g.::

  .. code-block:: bash

      perf stat -a -e imx8_ddr0/axid-read,axi_mask=0xMMMM,axi_id=0xDDDD/ cmd

      perf stat -a -e imx8_ddr0/axid-write,axi_mask=0xMMMM,axi_id=0xDDDD/ cmd

  NOTE: axi_mask is inverted in userspace(i.e. set bits are bits to mask), and

  .. note::

      axi_mask is inverted in userspace(i.e. set bits are bits to mask), and

      it will be reverted in driver automatically. so that the user can just specify

      axi_id to monitor a specific id, rather than having to specify axi_mask.

  e.g.::

  .. code-block:: bash

      perf stat -a -e imx8_ddr0/axid-read,axi_id=0x12/ cmd, which will monitor ARID=0x12

   :maxdepth: 1

   hisi-pmu

   imx-ddr

   qcom_l2_pmu

   qcom_l3_pmu

   arm-ccn