dts/bindings: Add binding for qemu,rv64 CPU

This patch add a 'rv64' binding which is QEMU RISC-V 64bit CPU.

Signed-off-by: Katsuhiro Suzuki <katsuhiro@katsuster.net>

dts/bindings/cpu/qemu,rv64.yaml

+# Copyright (c) 2020 Katsuhiro Suzuki
+# SPDX-License-Identifier: Apache-2.0
+
+description: >
+    This is a representation of QEMU rv64 CPU.
+
+compatible: "qemu,rv64"
+
+include: cpu.yaml