sh: dreamcast: Fix GAPS PCI bridge addressing

The G2-to-PCI bridge chip found in SEGA Dreamcast assumes P2 area
relative addresses.

Set the appropriate IOPORT base offset.

Tested-by: Paul Cercueil <paul@crapouillou.net>
Signed-off-by: Artur Rojek <contact@artur-rojek.eu>
Reviewed-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>
Link: https://lore.kernel.org/r/20240511191614.68561-2-contact@artur-rojek.eu


Signed-off-by: John Paul Adrian Glaubitz <glaubitz@physik.fu-berlin.de>