sched: add .init_pdata hook to the scheduler interface

sched: add .init_pdata hook to the scheduler interface

with the purpose of decoupling the allocation phase and
the initialization one, for per-pCPU data of the schedulers.

This makes it possible to perform the initialization later
in the pCPU bringup/assignement process, when more information
(for instance, the host CPU topology) are available. This,
for now, is important only for Credit2, but it can well be
useful to other schedulers.

Signed-off-by: Dario Faggioli <dario.faggioli@citrix.com>
Reviewed-by: Juergen Gross <jgross@suse.com>
Reviewed-by: George Dunlap <george.dunlap@citrix.com>