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

Add dt-bindings document for "qemu:pvpanic-mmio".
Acked-by: Mark Rutland <mark.rutland@arm.com>
Signed-off-by: Peng Hao <peng.hao2@zte.com.cn>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>

Documentation/devicetree/bindings/misc/pvpanic-mmio.txt

+* 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>;
+        };
+};
+