DEBUG: irq-prod/juno: Leverage an unused SPI

Note: it's unused by Linux and my board didn't catch fire by stressing it,
but it seems it's actually connected to something. I could only find a
Juno-r2 TRM (I'm using an r0), and that says that's the "HDLCD Controller
interrupt". There's a handful of "reserved" SPIs, but I have no idea if
those are any safer.

Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>