ARM SCPI MHU driver.

This driver uses the Message Handling Unit hardware to talk with
a System Controll Processor using the SCPI interface.

Required properties:
  - compatible: "arm,scpi-mhu"
  - reg: two pairs of register address and size data, first for the
    start of the MHU registers area, second for the payload area.
  - interrupts: Interrupt list for low priority and high priority
    channel, in that order.
  - #clock-cells: from common clock bindings; should be set to 1 if
    the SCP offers support for controlling oscillator frequencies, 0
    otherwise.
  - clock-output-names: List of names for the the oscillator output(s).


Examples:
   scp: scp@2b1f0000 {
	compatible = "arm,scpi-mhu";
	reg = <0x0 0x2b1f0000 0x0 0x10000>,   /* MHU registers */
	      <0x0 0x2e000000 0x0 0x10000>;   /* Payload area */
	interrupts = <0 36 4>,    /* low priority channel interrupt */
		     <0 35 4>,	  /* high priority channel interrupt */
		     <0 37 4>;    /* secure channel interrupt */
	#clock-cells = <1>;
	clock-output-names= "a57", "a53", "gpu", hdlcd0", "hdlcd1";
   };