The code mapping all PCI interrupts to a single CPU IRQ works but is
not trivial so document it in a comment.

Signed-off-by: BALATON Zoltan <balaton@eik.bme.hu>
Message-Id: <c25c0310510672b58466e795fd701e65e8f1ff97.1609636173.git.balaton@eik.bme.hu>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>