IPMI device

Required properties:
- compatible: should be one of ipmi-kcs, ipmi-smic, or ipmi-bt
- device_type: should be ipmi
- reg: Address and length of the register set for the device

Optional properties:
- interrupts: The interrupt for the device.  Without this the interface
	is polled.
- reg-size - The size of the register.  Defaults to 1
- reg-spacing - The number of bytes between register starts.  Defaults to 1
- reg-shift - The amount to shift the registers to the right to get the data
	into bit zero.

Example:

smic@fff3a000 {
	compatible = "ipmi-smic";
	device_type = "ipmi";
	reg = <0xfff3a000 0x1000>;
	interrupts = <0 24 4>;
	reg-size = <4>;
	reg-spacing = <4>;
};