cpu/hotplug: Online siblings when SMT control is turned on

Writing 'off' to /sys/devices/system/cpu/smt/control offlines all SMT
siblings. Writing 'on' merily enables the abilify to online them, but does
not online them automatically.

Make 'on' more useful by onlining all offline siblings.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>