riscv: qemu: enable booting on a second virtio device

QEMU RISC-V supports multiple virtio devices, but only tries to boot to
the first one. Enable support for a second virtio device, that is useful
for instance to boot on a disk image + an installer. Ideally that should
be made dynamic, but that's a first step.

Signed-off-by: Aurelien Jarno <aurelien@aurel32.net>
Reviewed-by: Leo Yu-Chi Liang <ycliang@andestech.com>

include/configs/qemu-riscv.h

@@ -23,6 +23,7 @@
 
 #define BOOT_TARGET_DEVICES(func) \
 	func(VIRTIO, virtio, 0) \
+	func(VIRTIO, virtio, 1) \
 	func(SCSI, scsi, 0) \
 	func(DHCP, dhcp, na)