nubus: move nubus to its own 32-bit address space

According to "Designing Cards and Drivers for the Macintosh Family" the Nubus
has its own 32-bit address space based upon physical slot addressing.

Move Nubus to its own 32-bit address space and then use memory region aliases
to map available slot and super slot ranges into the q800 system address
space via the Macintosh Nubus bridge.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-Id: <20210924073808.1041-13-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>