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

This is similar to the work already done for regs.

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

We're intentionally leaving some of the helpers in module scope here
to use some of the subroutines elsewhere later on when reworking
write_spi_dev().

Signed-off-by: Martí Bolívar <[email protected]>