docs: perf: Fix warning from 'make htmldocs' in hisi-pmu.rst

Building the 'htmldocs' target results in the following warning when
processing 'hisi-pmu.rst':

 | Documentation/admin-guide/perf/hisi-pmu.rst:107: ERROR: Unexpected indentation.

Fix the warning by converting all of the register bitfield lists into
proper bulleted lists and adjusting the indentation of the wrapping line
accordingly. At the same time, use an enumerated list to describe the
new PMUv2 functions.

Fixes: ea8d1c06 ("docs: perf: Add new description for HiSilicon UC PMU")
Reported-by: Stephen Rothwell <sfr@canb.auug.org.au>
Signed-off-by: Will Deacon <will@kernel.org>