riscv: qemu: Split devicetree files for qemu_riscv32/64

This uses QEMU virt which creates its own devicetree.

Copy the existing empty version of this file, so splitting the existing
qemu-virt into two, since anyone actually trying to use this will need a
different devicetree for 32- and 64-bit machines.

Tested-by: Heinrich Schuchardt <heinrich.schuchardt@canaonical.com>
Signed-off-by: Simon Glass <sjg@chromium.org>