hw/char: Extract serial-mm

hw/char: Extract serial-mm

hw/char/serial currently contains the implementation of both TYPE_SERIAL and
TYPE_SERIAL_MM. According to serial_class_init(), TYPE_SERIAL is an internal
class while TYPE_SERIAL_MM is used by numerous machine types directly. Let's
move the latter into its own module which makes the dependencies more obvious
and the code more tidy.

The includes and the dependencies have been converted mechanically except in the
hw/char directories which were updated manually. The result was compile-tested.
Now, only hw/char makes direct use of TYPE_SERIAL:

  # grep -r -e "select SERIAL" | grep -v SERIAL_
  hw/char/Kconfig:    select SERIAL
  hw/char/Kconfig:    select SERIAL
  hw/char/Kconfig:    select SERIAL
  hw/char/Kconfig:    select SERIAL
  hw/char/Kconfig:    select SERIAL

  # grep -r -e "/serial\\.h"
  include/hw/char/serial-mm.h:#include "hw/char/serial.h"
  hw/char/serial-pci-multi.c:#include "hw/char/serial.h"
  hw/char/serial.c:#include "hw/char/serial.h"
  hw/char/serial-isa.c:#include "hw/char/serial.h"
  hw/char/serial-pci.c:#include "hw/char/serial.h"

Tested-by: Andrew Jeffery <andrew@codeconstruct.com.au>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Signed-off-by: Bernhard Beschow <shentey@gmail.com>
Link: https://lore.kernel.org/r/20240905073832.16222-4-shentey@gmail.com
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>