Device tree configuration for i2c-ocores

Required properties:
- compatible      : "opencores,i2c-ocores"
                    "aeroflexgaisler,i2cmst"
                    "sifive,fu540-c000-i2c", "sifive,i2c0"
                    For Opencore based I2C IP block reimplemented in
                    FU540-C000 SoC. Please refer to sifive-blocks-ip-versioning.txt
                    for additional details.
- reg             : bus address start and address range size of device
- clocks          : handle to the controller clock; see the note below.
                    Mutually exclusive with opencores,ip-clock-frequency
- opencores,ip-clock-frequency: frequency of the controller clock in Hz;
                    see the note below. Mutually exclusive with clocks
- #address-cells  : should be <1>
- #size-cells     : should be <0>

Optional properties:
- interrupts      : interrupt number.
- clock-frequency : frequency of bus clock in Hz; see the note below.
                    Defaults to 100 KHz when the property is not specified
- reg-shift       : device register offsets are shifted by this value
- reg-io-width    : io register width in bytes (1, 2 or 4)
- regstep         : deprecated, use reg-shift above

Note
clock-frequency property is meant to control the bus frequency for i2c bus
drivers, but it was incorrectly used to specify i2c controller input clock
frequency. So the following rules are set to fix this situation:
- if clock-frequency is present and neither opencores,ip-clock-frequency nor
  clocks are, then clock-frequency specifies i2c controller clock frequency.
  This is to keep backwards compatibility with setups using old DTB. i2c bus
  frequency is fixed at 100 KHz.
- if clocks is present it specifies i2c controller clock. clock-frequency
  property specifies i2c bus frequency.
- if opencores,ip-clock-frequency is present it specifies i2c controller
  clock frequency. clock-frequency property specifies i2c bus frequency.

Examples:

	i2c0: ocores@a0000000 {
		#address-cells = <1>;
		#size-cells = <0>;
		compatible = "opencores,i2c-ocores";
		reg = <0xa0000000 0x8>;
		interrupts = <10>;
		opencores,ip-clock-frequency = <20000000>;

		reg-shift = <0>;	/* 8 bit registers */
		reg-io-width = <1>;	/* 8 bit read/write */

		dummy@60 {
			compatible = "dummy";
			reg = <0x60>;
		};
	};
or
	i2c0: ocores@a0000000 {
		#address-cells = <1>;
		#size-cells = <0>;
		compatible = "opencores,i2c-ocores";
		reg = <0xa0000000 0x8>;
		interrupts = <10>;
		clocks = <&osc>;
		clock-frequency = <400000>; /* i2c bus frequency 400 KHz */

		reg-shift = <0>;	/* 8 bit registers */
		reg-io-width = <1>;	/* 8 bit read/write */

		dummy@60 {
			compatible = "dummy";
			reg = <0x60>;
		};
	};