perf/arm_cspmu: Simplify attribute groups

The attribute group array itself is always the same, so there's no
need to allocate it separately. Storing it directly in our instance
data saves memory and gives us one less point of failure.

Reviewed-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Reviewed-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Link: https://lore.kernel.org/r/cf12b803114b0815438833fcb2495f20f2007761.1706718007.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>