ACPI: Add OSI for CPU runtime changes to present and enabled

Firmware can make an eject-request call to the OS, and the OS may take
a CPU offline, but this doesn't tell the firmware whether the OS actually
supports making CPUs not present.

The same goes for disabling CPUs, firmware can't know if the OS
supports these bits being changed at runtime.

Add OSI strings for both.

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