arm64/sve: Add Perf extensions documentation

Document that the VG register is available in Perf samples

Signed-off-by: James Clark <james.clark@arm.com>
Reviewed-by: Mark Brown <broonie@kernel.org>
Link: https://lore.kernel.org/r/20220901132658.1024635-3-james.clark@arm.com


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