ACPI: add support to register CPUs based on the _STA enabled bit

acpi_processor_get_info() registers all present CPUs. Registering a
CPU is what creates the sysfs entries and triggers the udev
notifications.

arm64 virtual machines that support 'virtual cpu hotplug' use the
enabled bit to indicate whether the CPU can be brought online, as
the existing ACPI tables require all hardware to be described and
present.

If firmware describes a CPU as present, but disabled, skip the
registration. Such CPUs are present, but can't be brought online for
whatever reason. (e.g. firmware/hypervisor policy).

Once firmware sets the enabled bit, the CPU can be registered and
brought online by user-space. Online CPUs, or CPUs that are missing
an _STA method must always be registered.

Signed-off-by: James Morse <james.morse@arm.com>