MIPS: ath79: add initial support for DPT-Module

The following features are supported:

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

Links:

  * https://dptechnics.com/shop/index.php?route=product/product&path=59&product_id=50
  * https://dptechnics.com/shop/index.php?route=product/product&path=59&product_id=63

Signed-off-by: Antony Pavlov <antonynpavlov@gmail.com>
Cc: Daan Pape <daan@dptechnics.com>
Cc: linux-mips@linux-mips.org
Cc: devicetree@vger.kernel.org
Patchwork: https://patchwork.linux-mips.org/patch/12886/
Signed-off-by: Ralf Baechle <ralf@linux-mips.org>