ACPI: Enable Platform Runtime Mechanism(PRM) support on ARM64

There is interest to make use of PRM(Platform Runtime Mechanism) even on
ARM64 ACPI platforms. Allow PRM to be enabled on ARM64 platforms. It will
be enabled by default as on x86_64.

Signed-off-by: Sudeep Holla <sudeep.holla@arm.com>
Acked-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>
Signed-off-by: Ard Biesheuvel <ardb@kernel.org>