MIPS: ath79: add initial support for Dragino MS14 (Dragino 2)

MIPS: ath79: add initial support for Dragino MS14 (Dragino 2)

The following features are supported:

  * UART;
  * SPI-flash;
  * USB host;
  * GPIO keys and LEDs.

Links:

    * http://www.dragino.com/products/mother-board/item/71-ms14-p.html
    * https://wiki.openwrt.org/toh/dragino/ms14

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Alban Bedel <albeu@free.fr>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12882/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>