dt-bindings: net: ks8995: add bindings documentation for ks8995

Micrel KS8995 SPI controlled Ethernet Switch families

Required properties (according to spi-bus.txt):

- compatible: either "micrel,ks8995", "micrel,ksz8864" or "micrel,ksz8795"

Optional properties:

- reset-gpios : phandle of gpio that will be used to reset chip during probe

Example:

spi-master {

	...

	switch@0 {

		compatible = "micrel,ksz8795";

		reg = <0>;

		spi-max-frequency = <50000000>;

		reset-gpios = <&gpio0 46 GPIO_ACTIVE_LOW>;

	};

};