hw/openrisc/openrisc_sim: Use IRQ splitter when connecting UART

hw/openrisc/openrisc_sim: Use IRQ splitter when connecting UART

Currently the OpenRISC SMP configuration only supports 2 cores due to
the UART IRQ routing being limited to 2 cores.  As was done in commit
1eeffbeb11 ("hw/openrisc/openrisc_sim: Use IRQ splitter when connecting
IRQ to multiple CPUs") we can use a splitter to wire more than 2 CPUs.

This patch moves serial initialization out to it's own function and
uses a splitter to connect multiple CPU irq lines to the UART.

Signed-off-by: Stafford Horne <shorne@gmail.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>