ar71xx: add support for RB-941-2nD

ar71xx: add support for RB-941-2nD

Add full support for Mikrotik RB-941-2nD (hAP lite)

Original patch by Sergey Sergeev <adron@yapic.net> and
more information is available here:
https://wiki.openwrt.org/toh/mikrotik/rb941_2nd
I updated and adapted the patch to apply cleanly to LEDE trunk
and added proper numbering for the switch ports (matching case
labels).
Tested working on actual hardware with the information
provided in the above webpage. Sysupgrade works.

Signed-off-by: Thibaut VARENE <hacks@slashdirt.org>
Signed-off-by: Felix Fietkau <nbd@nbd.name>