Add support for Realtek RTL838x/RTL839x switch SoCs

The RTL838x/839x family of SoCs are Realtek switches with an embedded
MIPS core.

* RTL838x - 500MHz 4kce single core - 1Gbit ports and L2 features
* RTL839x - 700MHz 34Kc single core - 1Gbit ports and L2 features

These switches, depending on the exact part number, will have anywhere
between 8 and 52 ports. The MIPS core is wired to a switch cpu port which
has a tagging feature allowing us to make use of the DSA subsystem.
The SoCs are somewhat basic in certain areas, getting better with more
advanced features on newer series.

The switch functionality is MMIO-mapped via a large MFD region.

The SoCs have the following peripherals
* ethernet
* switch
* uart - ns16550a
* spi-flash interface
* gpio
* wdt
* led

The code was derived from various vendor SDKs based on Linux v2.6
kernels.

This patchset allows us to boot RTL838x/RTL839x units with basic support.
Most of the other drivers are already written and functional, and work to
get them upstream is already in progress.

Signed-off-by: Birger Koblitz <mail@birger-koblitz.de>
Signed-off-by: Bert Vermeulen <bert@biot.com>
Signed-off-by: John Crispin <john@phrozen.org>
Signed-off-by: Sander Vanheule <sander@svanheule.net>
Signed-off-by: Thomas Bogendoerfer <tsbogend@alpha.franken.de>