scripts: edtlib: Add a Device.children attribute with child Devices

API oversight. This was meant to be there all along together with
Device.parent, for navigating the devicetree, but since a need for it
never came up in gen_defines.py, it got overlooked.

Devices are just devicetree nodes augmented with binding information and
some interpretation of devicetree properties. I wonder if the name
should be changed to something like edtlib.Node to make that clearer.
Calling something like a flash partition a "device" is a bit weird, as
Galak pointed out.

I think I went with Device originally to avoid confusion with
dtlib.Node, but since edtlib users don't directly interact with dtlib,
it might not be that confusing in practice.

Piggyback some documentation clarifications.

Signed-off-by: Ulf Magnusson <Ulf.Magnusson@nordicsemi.no>