ar71xx: initial support for Mikrotik RB751G-2HnD and RB751U-2HnD

Great collaboration on these two fantastic routers on the openwrt forum:
https://forum.openwrt.org/viewtopic.php?id=32320


Above all kudos to aryufan. Well done and thank you everyone else who
contributed.

To-Do: LED for wlan is not yet activated
To-Do: TxPower over 20dBm (RB751U) or 22dBm (RB751G) are not accepted by
the router

Signed-off-by: Hanno Schupp <hanno.schupp@gmail.com>
[juhosg: fix line wrapped patch, fix checkpatch errors, add missing
kernel patches]

SVN-Revision: 31208