It can be useful to know what a node's index is in its parent's list
of children. This information is now available to C via
gen_defines.py, but no user-facing macros are available to access it.
Add a macro which exposes this information to users via devicetree.h.
Some APIs want to build on devicetree.h by creating some derived
structure for each of a node's children. It can therefore be
convenient to use each child's index in the list of children as an
identifier for the child.
Some concrete and common examples are "gpio-keys" and "gpio-leds",
which allow you to define arbitrary numbers of keys and LEDs as child
nodes of nodes with those compatibles. Derived APIs can use a key or
LED node's index in its list of parents as a way to identify which of
several structures is relevant to a particular controlled key or LED.
These are just examples, though -- the feature added here makes no
assumptions about where it's being used.
Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>
869ae2b5ac