mips: dts: realtek: Add RTL838x SoC peripherals

Add some of the SoC's CPU peripherals currently supported:
  - GPIO controller with support for 24 GPIO lines, although not all
    lines are brought out to pads on the SoC package. These lines can
    generate interrupts from external sources.
  - Watchdog which can be used to restart the SoC if no external restart
    logic is present.
  - SPI controller, primarily used to access NOR flash

Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>