ramips: add support for Zyxel NBG-419N2 (WAP3205v2)

applied bb-final-ramips-add-zyxel-nbg-419n2.patch from
123serge123, found at https://yadi.sk/d/1ZV0lKJwbTE65;
see https://forum.openwrt.org/viewtopic.php?pid=246905#p246905

,
modified slightly to fit to CC release and to new lede build
system: image/rt305x.mk include file is used now
changed NBG-419N2.dts format to fit style of other dts files

Signed-off-by: Klaus <k-laus@quantentunnel.de>