PCI: keystone: Add error IRQ handler

Keystone PCI hardware generates error interrupts at RC using a platform IRQ
instead of a standard MSI or legacy IRQ.  Add a simple error handler that
logs the fatal interrupt status to the console.

[bhelgaas: s/node/dev->of_node/, tidy comments, return irqreturn_t directly]
Signed-off-by: Murali Karicheri <m-karicheri2@ti.com>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Acked-by: Rob Herring <robh@kernel.org>
CC: Pawel Moll <pawel.moll@arm.com>
CC: Mark Rutland <mark.rutland@arm.com>
CC: Ian Campbell <ijc+devicetree@hellion.org.uk>
CC: Kumar Gala <galak@codeaurora.org>