HMP: Select least-loaded CPU when performing HMP Migrations

HMP: Select least-loaded CPU when performing HMP Migrations

The reference patch set always selects the first CPU in an HMP
domain as a migration target. In busy situations, this means that
the migrated thread cannot make immediate use of an idle CPU but
must share a busy one until the load balancer runs across the big
domain.

This patch uses the hmp_domain_min_load function introduced in
global balancing to figure out which of the CPUs is the least busy
and selects that as a migration target - in both directions.

This essentially implements a task-spread strategy and is intended
to maximise performance of migrated threads but is likely
to use more power than the packing strategy previously employed.

Signed-off-by: Chris Redpath <chris.redpath@arm.com>