scripts: devicetree.py: get alternate labels from dt

Allow use of new element of dtc grammar called overriding nodes:
i2cexp: &i2c2  {};

It allows a node to assign an alternate label to a node that
could be generic and used for adapter boards.
This commit is a derivative of a dtc commit from dtc v1.4.2 [1]

[1] https://bit.ly/2GFLLOa



Signed-off-by: Erwan Gouriou <erwan.gouriou@linaro.org>