MIPS: ath79: add initial support for TP-LINK MR3020

MIPS: ath79: add initial support for TP-LINK MR3020

The following features are supported:

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

Links:

  * http://www.tp-link.com/en/products/details/?model=TL-MR3020
  * http://wiki.openwrt.org/toh/tp-link/tl-mr3020
  * https://wikidevi.com/wiki/TP-LINK_TL-MR3020

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/12880/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>