devicetree,xen: add xen,shared-memory binding

Introduce a device tree binding for Xen reserved-memory regions. They
are used to share memory across VMs from the VM config files. (See
static_shm config option.)
Signed-off-by: Stefano Stabellini <stefanos@xilinx.com>
Cc: julien.grall@arm.com
Cc: devicetree@vger.kernel.org
Cc: robh+dt@kernel.org
Cc: mark.rutland@arm.com
Cc: xen-devel@lists.xen.org
Signed-off-by: Rob Herring <robh@kernel.org>

Documentation/devicetree/bindings/reserved-memory/xen,shared-memory.txt

+* Xen hypervisor reserved-memory binding
+
+Expose one or more memory regions as reserved-memory to the guest
+virtual machine. Typically, a region is configured at VM creation time
+to be a shared memory area across multiple virtual machines for
+communication among them.
+
+For each of these pre-shared memory regions, a range is exposed under
+the /reserved-memory node as a child node. Each range sub-node is named
+xen-shmem@<address> and has the following properties:
+
+- compatible:
+	compatible = "xen,shared-memory-v1"
+
+- reg:
+	the base guest physical address and size of the shared memory region
+
+- xen,offset: (borrower VMs only)
+	64 bit integer offset within the owner virtual machine's shared
+	memory region used for the mapping in the borrower VM.
+
+- xen,id:
+	a string that identifies the shared memory region as specified in
+	the VM config file