dts: gen_defines: emit more for-each helpers

Add some helper macros that work similarly to the
'DT_FOREACH_OKAY_INST_<compat>(fn)' macros, except they give 'fn'
node identifiers in their expansion instead of instance numbers.

This makes it possible to add for-each APIs to devicetree.h that work
on an arbitrary compatible, not just DT_DRV_COMPAT.

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