MAINTAINERS: Add CPU HOTPLUG entry

Document the status quo and add myself and Peter as CPU hotplug
maintainers.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Randy Dunlap <rdunlap@infradead.org>
Link: https://lore.kernel.org/r/87o7ll6ogo.ffs@tglx