perf/arm_cspmu: Clean up ACPI dependency

Build-wise, the ACPI dependency consists of only a couple of things
which could probably stand being factored out into ACPI helpers anyway.
However for the immediate concern of working towards Devicetree support
here, it's easy enough to make a few tweaks to contain the affected code
locally, such that we can relax the Kconfig dependency.

Reviewed-and-Tested-by: Suzuki K Poulose <suzuki.poulose@arm.com>
Reviewed-by: Ilkka Koskinen <ilkka@os.amperecomputing.com>
Signed-off-by: Robin Murphy <robin.murphy@arm.com>
Link: https://lore.kernel.org/r/9d126711c7498b199b3e6f5cf48ca60ffb9df54c.1685983270.git.robin.murphy@arm.com
Signed-off-by: Will Deacon <will@kernel.org>