ice: Add an option to pre-allocate memory for ice_sched_node

devlink-rate API requires a priv object to be allocated when node still
doesn't have a parent. This is problematic, because ice_sched_node can't
be currently created without a parent.

Add an option to pre-allocate memory for ice_sched_node struct. Add
new arguments to ice_sched_add() and ice_sched_add_elems() that allow
for pre-allocation of memory for ice_sched_node struct.

Signed-off-by: Michal Wilczynski <michal.wilczynski@intel.com>
Signed-off-by: Jakub Kicinski <kuba@kernel.org>