scripts: devicetree: Handle mixed case node names

This patch fixes an issue with mixed case node names.  Node names
containing upper case letters were causing key match errors during
post processing.  This patch maintains the case of the node names.

Change-Id: I153a186fa09dcf958c9de55b578dbc63e615a076
Signed-off-by: Andy Gross <andy.gross@linaro.org>