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>