i2c: Add DT bindings for Xscale I2C masters

This adds a device tree binding for Intel XScale I2C
masters. We define compatible strings for the iop3xx and
ixp4xx chip families.
Signed-off-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Wolfram Sang <wsa@the-dreams.de>

Documentation/devicetree/bindings/i2c/i2c-xscale.txt

+i2c Controller on XScale platforms such as IOP3xx and IXP4xx
+
+Required properties:
+- compatible : Must be one of
+  "intel,iop3xx-i2c"
+  "intel,ixp4xx-i2c";
+- reg
+- #address-cells = <1>;
+- #size-cells = <0>;
+
+Optional properties:
+- Child nodes conforming to i2c bus binding
+
+Example:
+
+i2c@c8011000 {
+	compatible = "intel,ixp4xx-i2c";
+	reg = <0xc8011000 0x18>;
+	interrupts = <33 IRQ_TYPE_LEVEL_LOW>;
+};