scripts: gen_defines: re-work write_regs() with augmented nodes

Group the macros together by namespace rather than putting all the
BASE_ADDRESS macros together and all the SIZE macros together. E.g.,
all the DT_INST_<x> namespace macros for each node now appear
consecutively.

Add a comment making it clear that this output comes from "regs",
since "BASE_ADDRESS" and "SIZE" are not property names.

Other than the order in which they appear and comments, the output
before and after this patch should be exactly the same.

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