RISC-V machine with device-tree, 16550a UART and VirtIO MMIO.
The following machine is implemented:

- 'virt'; CLINT, PLIC, 16550A UART, VirtIO MMIO, device-tree
Acked-by: NRichard Henderson <richard.henderson@linaro.org>
Signed-off-by: NPalmer Dabbelt <palmer@sifive.com>
Signed-off-by: NMichael Clark <mjc@sifive.com>