docs: perf: Minimal introduction the the CXL PMU device and driver

Very basic introduction to the device and the current driver support
provided. I expect to expand on this in future versions of this patch
set.

Reviewed-by: Dave Jiang <dave.jiang@intel.com>
Reviewed-by: Kan Liang <kan.liang@linux.intel.com>
Signed-off-by: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Link: https://lore.kernel.org/r/20230526095824.16336-6-Jonathan.Cameron@huawei.com


Signed-off-by: Dan Williams <dan.j.williams@intel.com>