spi: jcore: add J-Core SPI master bindings

J-Core SPI master

Required properties:

- compatible: Must be "jcore,spi2".

- reg: Memory region for registers.

- #address-cells: Must be 1.

- #size-cells: Must be 0.

Optional properties:

- clocks: If a phandle named "ref_clk" is present, SPI clock speed

  programming is relative to the frequency of the indicated clock.

  Necessary only if the input clock rate is something other than a

  fixed 50 MHz.

- clock-names: Clock names, one for each phandle in clocks.

See spi-bus.txt for additional properties not specific to this device.

Example:

spi@40 {

	compatible = "jcore,spi2";

	#address-cells = <1>;

	#size-cells = <0>;

	reg = <0x40 0x8>;

	spi-max-frequency = <25000000>;

	clocks = <&bus_clk>;

	clock-names = "ref_clk";

}