This adds support for detecting this model board and registers some LEDs
and buttons.

There are two uncommon things regarding this device:
1) It can use two different "board_id" ID values.
   Unit I have uses "U12H139T00_NETGEAR" value. This magic is also used
   in firmware file header. There are two reports (one from an OpenWrt
   user) of a different "U12H139T50_NETGEAR" magic though.
2) Power LEDs share GPIOs with buttons.
   Amber one seems to share GPIO 2 with WPS button and green one seems
   to share GPIO 3 with reset button. It remains unknown how to support
   them and handle buttons at the same time. For that reason they aren't
   added to the list of supported LEDs.
Signed-off-by: Rafał Miłecki <rafal@milecki.pl>
Cc: Ralf Baechle <ralf@linux-mips.org>
Cc: Hauke Mehrtens <hauke@hauke-m.de>
Cc: linux-mips@linux-mips.org
Patchwork: https://patchwork.linux-mips.org/patch/19004/Signed-off-by: James Hogan <jhogan@kernel.org>