sched/topology: Record number of cores in sched group

When balancing sibling domains that have different number of cores,
tasks in respective sibling domain should be proportional to the
number of cores in each domain. In preparation of implementing such a
policy, record the number of cores in a scheduling group.

Signed-off-by: Tim Chen <tim.c.chen@linux.intel.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lore.kernel.org/r/04641eeb0e95c21224352f5743ecb93dfac44654.1688770494.git.tim.c.chen@linux.intel.com