docs: perf: Add description for Alibaba's T-Head PMU driver

Alibaba's T-Head SoC implements uncore 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: Jonathan Cameron <Jonathan.Cameron@huawei.com>
Reviewed-by: Baolin Wang <baolin.wang@linux.alibaba.com>
Link: https://lore.kernel.org/r/20220914022326.88550-2-xueshuai@linux.alibaba.com


Signed-off-by: Will Deacon <will@kernel.org>