riscv: qemu: use device tree passed by prior boot stage

QEMU provides a device tree, which is passed to U-Boot using register
a1. We are now able to directly select the device tree with the
configuration CONFIG_OF_PRIOR_STAGE. Replace the hard-coded address in
qemu-riscv with it.

Signed-off-by: Lukas Auer <lukas.auer@aisec.fraunhofer.de>
Reviewed-by: Bin Meng <bmeng.cn@gmail.com>
Reviewed-by: Rick Chen <rick@andestech.com>