When walking the devices rbtree to insert a node, we must keep track of the
parent node when we descend. If we skip this step, we always insert new
nodes with a NULL parent, bypassing __rb_insert()s rebalance code.

Things get worse when we come to walk the tree, as we can't move up a
level. This isn't a problem in practice, as all devices appear to be
inserted in-order, so our rbtree is actually a monochrome linked list.

Signed-off-by: James Morse <james.morse@arm.com>
Signed-off-by: Will Deacon <will.deacon@arm.com>