Sort subnodes of the root node alphanumerically.

This is part of an ongoing effort to provide consistent node
order in the DT of Renesas SoCs to improve maintainability.

This should not have any run-time effect.
Signed-off-by: Yoshihiro Kaneko <ykaneko0929@gmail.com>
Signed-off-by: Simon Horman <horms+renesas@verge.net.au>