hw/openrisc: support 4 serial ports in or1ksim

The 8250 serial controller supports 4 serial ports, so wire them all up,
so that we can have more than one basic I/O channel.

Cc: Stafford Horne <shorne@gmail.com>
Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>
[smh:Fixup indentation and lines over 80 chars]
Signed-off-by: Stafford Horne <shorne@gmail.com>