sun4m: don't connect two qemu_irqs directly to the same input

sun4m: don't connect two qemu_irqs directly to the same input

The sun4m board code connects both of the IRQ outputs of each ESCC to the
same slavio input qemu_irq. Connecting two qemu_irqs outputs directly to the
same input is not valid as it produces subtly wrong behaviour (for instance
if both the IRQ lines are high, and then one goes low, the PIC input will see
this as a high-to-low transition even though the second IRQ line should still
be holding it high).

This kind of wiring needs an explicitly created OR gate; add one.

Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>
Message-Id: <20201219111934.5540-1-mark.cave-ayland@ilande.co.uk>
Reviewed-by: Artyom Tarasenko <atar4qemu@gmail.com>
Signed-off-by: Mark Cave-Ayland <mark.cave-ayland@ilande.co.uk>