arm64: Declare ACPI parking protocol CPU operation if needed

It's obvious we needn't declare the corresponding CPU operation when
CONFIG_ARM64_ACPI_PARKING_PROTOCOL is disabled, even it doesn't cause
any compiling warnings.

Signed-off-by: Gavin Shan <gshan@redhat.com>
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>