Documentation / CPU hotplug: Rephrase the outdated description for MADT entries

More than 256 entries in ACPI MADT is supported from ACPI 3.0, so the
information in should be Documentation/cpu-hotplug.txt updated.

[rjw: Changelog]
Suggested-by: Rafael J. Wysocki <rjw@sisk.pl>
Signed-off-by: Hanjun Guo <hanjun.guo@linaro.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>