escc: introduce a selector for the register bit

On Sparc and PowerMac, the bit 0 of the address selects the register
type (control or data) and bit 1 selects the channel (B or A).

On m68k Macintosh and NeXTcube, the bit 0 selects the channel and
bit 1 the register type.

This patch introduces a new parameter (bit_swap) to the device interface
to indicate bits usage must be swapped between registers and channels.

For the moment all the machines use the bit 0, but this change will be
needed to emulate the Quadra 800 or NeXTcube machine.

Signed-off-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Hervé Poussineau <hpoussin@reactos.org>
[thh: added NeXTcube to the patch description]
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Tested-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Message-Id: <20190831074519.32613-5-huth@tuxfamily.org>
Signed-off-by: Thomas Huth <huth@tuxfamily.org>

include/hw/char/escc.h

@@ -51,6 +51,7 @@ typedef struct ESCCState {
 
     struct ESCCChannelState chn[2];
     uint32_t it_shift;
+    bool bit_swap;
     MemoryRegion mmio;
     uint32_t disabled;
     uint32_t frequency;