m68k: Add NeXTcube keyboard device

m68k: Add NeXTcube keyboard device

It is likely still quite incomplete (e.g. mouse and interrupts are not
implemented yet), but it is good enough for keyboard input at the firmware
monitor.
This code has been taken from Bryce Lanham's GSoC 2011 NeXT branch at

 https://github.com/blanham/qemu-NeXT/blob/next-cube/hw/next-kbd.c

and altered to fit the latest interface of the current QEMU (e.g. to use
memory_region_init_io() instead of cpu_register_physical_memory()).

Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190831074519.32613-3-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>