Documentation: hisi-pmu: Add introduction to HiSilicon V3 PMU

Documentation: hisi-pmu: Add introduction to HiSilicon V3 PMU

Some of HiSilicon V3 PMU hardware is divided into parts to fulfill the
job of monitoring specific parts of a device.  Add description on that
as well as the newly added ext option for L3C PMU.

Acked-by: Jonathan Cameron <jonathan.cameron@huawei.com>
Signed-off-by: Yushan Wang <wangyushan12@huawei.com>
Reviewed-by: Yicong Yang <yangyicong@hisilicon.com>
Signed-off-by: Will Deacon <will@kernel.org>