swim: split into separate IWM and ISM register blocks

swim: split into separate IWM and ISM register blocks

The swim chip provides an implementation of both Apple's IWM and ISM floppy disk
controllers. Split the existing implementation into separate register banks for
each controller, whilst also switching the IWM registers from 16-bit to 8-bit
as implemented in real hardware.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Message-ID: <20231004083806.757242-13-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>