apm821xx: provide legacy interrupts for PCIe in DT

apm821xx: provide legacy interrupts for PCIe in DT

Devices with PCIe-Switches like the WNDR4700, MR24 and WNDAP660
need to have the interrupts property specified in the device-tree
for the legacy pci interrupt signaling method to work.

If the proper interrupt value is not specified, the default INTA
IRQ 12 is taken for all devices. This is especially bad, if the
device is setup to use INTC, because these interrupts will not
be serviced.

Russell Senior reported his experience on the MR24:
"The symptom is client devices can't see the beacons.
Wifi ifaces appear, can scan and hear other networks,
but clients can't see the MR24's SSIDs."

(The interrupts-property on the WNDAP620 was optional since it
uses INTA by default. Likewise the MX60W is in the same category)

Reported-by: Russell Senior <russell@personaltelco.net>
Signed-off-by: Christian Lamparter <chunkeey@gmail.com>