riscv/virt: Add syscon reboot and poweroff DT nodes

The SiFive test device found on virt machine can be used by
generic syscon reboot and poweroff drivers available in Linux
kernel.

This patch updates FDT generation in virt machine so that
Linux kernel can probe and use generic syscon drivers.

Signed-off-by: Anup Patel <anup.patel@wdc.com>
Reviewed-by: Palmer Dabbelt <palmerdabbelt@google.com>
Signed-off-by: Palmer Dabbelt <palmerdabbelt@google.com>