MIPS: ath79: add initial support for Onion Omega

MIPS: ath79: add initial support for Onion Omega

The following features are supported:

  * UART;
  * SPI-flash;
  * USB host;
  * GPIO key and LED.

Please see https://onion.io/omega for details.

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Alban Bedel <albeu@free.fr>
Cc: L. D. Pinney <ldpinney@gmail.com>
Cc: Boken Lin <bl@onion.io>
Cc: Jacky Huang <huangfangcheng@163.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12884/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>