nubus: add support for slot IRQs

nubus: add support for slot IRQs

Each Nubus slot has an IRQ line that can be used to request service from the
CPU. Connect the IRQs to the Nubus bridge so that they can be wired up using qdev
gpios accordingly, and introduce a new nubus_set_irq() function that can be used
by Nubus devices to control the slot IRQ.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Reviewed-by: Laurent Vivier <laurent@vivier.eu>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Message-Id: <20210924073808.1041-19-mark.cave-ayland@ilande.co.uk>
Signed-off-by: Laurent Vivier <laurent@vivier.eu>