support multiple UARTs in qemu-riscv-virt by default
Goal
Add at least one more UART to qemu-riscv-virt.
Technical details
Is there a specific reason why qemu-riscv-virt just has one UART? Or is this just copied from the ARM virt? Having at least one other UART would allow to have a system console and also an independent UART for a VM or user application. This also relaxes the need to multiplex things on the one UART that exists.