hw/block/fdc: Declare shared prototypes in fdc-internal.h

We want to extract ISA/SysBus code from the generic fdc.c file.
First, declare the prototypes we will access from the new units
into a new local header: "fdc-internal.h".

Acked-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Acked-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-id: 20210614193220.2007159-4-philmd@redhat.com
Signed-off-by: John Snow <jsnow@redhat.com>