ARM: sched: Avoid empty 'slow' HMP domain

ARM: sched: Avoid empty 'slow' HMP domain

On homogeneous (non-heterogeneous) systems all CPUs will be declared
'fast' and the slow cpu list will be empty. In this situation we need to
avoid adding an empty slow HMP domain otherwise the scheduler code will
blow up when it attempts to move a task to the slow domain.

Signed-off-by: Jon Medhurst <tixy@linaro.org>