docs: perf: Add description for Synopsys DesignWare PCIe PMU driver

docs: perf: Add description for Synopsys DesignWare PCIe PMU driver

Alibaba's T-Head Yitan 710 SoC includes Synopsys' DesignWare Core PCIe
controller which implements PMU for performance and functional debugging to
facilitate system maintenance.

Document it to provide guidance on how to use it.

Signed-off-by: Shuai Xue <xueshuai@linux.alibaba.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Reviewed-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Yicong Yang <yangyicong@hisilicon.com>
Tested-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Link: https://lore.kernel.org/r/20231208025652.87192-2-xueshuai@linux.alibaba.com
Signed-off-by: Will Deacon <will@kernel.org>