scripts/dts/gen_defines.py: exclude initializer lists from conf file

Things that parse generated_dts_board.conf can't deal with entries
like:

    DT_GPIO_KEYS_BUTTON_1_GPIOS={"GPIO_0", 14, 256}

so keep them from being added there.

Signed-off-by: Peter A. Bigot <pab@pabigot.com>