arm: riscv: qemu: Explain how to extract the generated dt

QEMU currently generates a devicetree for use with U-Boot. Explain how to
obtain it.

Also explain how to merge it to produce a devicetree with the U-Boot
features included.

Signed-off-by: Simon Glass <sjg@chromium.org>
diff --git a/doc/board/emulation/qemu-riscv.rst b/doc/board/emulation/qemu-riscv.rst
index 4b8e104..3409fff 100644
--- a/doc/board/emulation/qemu-riscv.rst
+++ b/doc/board/emulation/qemu-riscv.rst
@@ -13,6 +13,9 @@
 the VirtIO standard networking and block storage devices. It has CLINT, PLIC,
 16550A UART devices in addition to VirtIO and it also uses device-tree to pass
 configuration information to guest software. It implements RISC-V privileged
+
+See :doc:`../../develop/devicetree/dt_qemu` for information on how to see
+the devicetree actually generated by QEMU.
 architecture spec v1.10.
 
 Building U-Boot