platform: msm_bus: Fix aggregation param parsing for nodes

Fix the DT parsing of the aggregation parameters for nodes. A node
can have no aggregation scheme override but could still override the
util and vrail comp factors.

Change-Id: Id7e77dd3e6680d357a9e63b1333739d52050a580
Signed-off-by: Girish Mahadevan <girishm@codeaurora.org>