hw/sparc64/ebus: Access memory regions via pci_address_space_io()

PCI functions are plugged on a PCI bus. They can only access
external memory regions via the bus.

Signed-off-by: Philippe Mathieu-Daudé <philmd@linaro.org>
Reviewed-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20231011185954.10337-5-philmd@linaro.org>