net: dsa: Add bindings for Realtek SMI DSAs

The Realtek SMI family is a set of DSA chips that provide
switching in routers. This binding just follows the pattern
set by other switches but with the introduction of an embedded
irqchip to demux and handle the interrupts fired by the single
line from the chip.

This interrupt construction is similar to how we handle
interrupt controllers inside PCI bridges etc.

Cc: Antti Seppälä <a.seppala@gmail.com>
Cc: Roman Yeryomin <roman@advem.lv>
Cc: Colin Leitner <colin.leitner@googlemail.com>
Cc: Gabor Juhos <juhosg@openwrt.org>
Cc: Florian Fainelli <f.fainelli@gmail.com>
Cc: devicetree@vger.kernel.org
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: David S. Miller <davem@davemloft.net>