hw/acpi/piix4: use qdev gpio to wire up sci_irq

hw/acpi/piix4: use qdev gpio to wire up sci_irq

Introduce piix4_pm_init() instance init function and use it to
initialise the separate qdev gpio for the SCI IRQ.

The sci_irq can now be wired up directly using a qdev gpio instead
of having to set the IRQ externally in piix4_pm_initfn().

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20220528091934.15520-9-mark.cave-ayland@ilande.co.uk>
[PMD: Partially squash 20220528091934.15520-8-mark.cave-ayland@ilande.co.uk]
Reviewed-by: Bernhard Beschow <shentey@gmail.com>
Signed-off-by: Philippe Mathieu-Daudé <f4bug@amsat.org>