gen_defines: add helpers for iterating over the entire tree

Add two helper macros:

- DT_FOREACH_HELPER
- DT_FOREACH_OKAY_HELPER

These are internal generated macros which will be used to define
devicetree.h APIs for iterating over every node in the tree, and every
node in the tree with status "okay", respectively.

We can use these new APIs to implement the functionality in
device_extern.h natively from device.h without requiring a generated
header. It will also be useful for other purposes later on.

Signed-off-by: Martí Bolívar <marti.bolivar@nordicsemi.no>