perf/arm-cmn: Improve build-time assertion

These days we can use static_assert() in the logical place rather than
jamming a BUILD_BUG_ON() into the nearest function scope.

Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/224ee8286f299100f1c768edb254edc898539f50.1725296395.git.robin.murphy@arm.com


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