perf: Simplify group validation

All of these drivers copy a pattern of actively policing cross-PMU
groups, which is redundant since commit bf480f93 ("perf/core: Don't
allow grouping events from different hw pmus"). Clean up these checks to
simplfy matters, especially for thunderx2 which can reduce right down to
trivial counting.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>