dt-bindings: misc/pvpanic: add document for pvpanic-mmio

* QEMU PVPANIC MMIO Configuration bindings

QEMU's emulation / virtualization targets provide the following PVPANIC

MMIO Configuration interface on the "virt" machine.

type:

- a read-write, 16-bit wide data register.

QEMU exposes the data register to guests as memory mapped registers.

Required properties:

- compatible: "qemu,pvpanic-mmio".

- reg: the MMIO region used by the device.

  * Bytes 0x0  Write panic event to the reg when guest OS panics.

  * Bytes 0x1  Reserved.

Example:

/ {

        #size-cells = <0x2>;

        #address-cells = <0x2>;

        pvpanic-mmio@9060000 {

                compatible = "qemu,pvpanic-mmio";

                reg = <0x0 0x9060000 0x0 0x2>;

        };

};