powerpc: Update currituck pci/usb fixup for new board revision

The currituck board uses a different IRQ for the pci usb host
controller depending on the board revision. This patch adds support
for newer board revisions by retrieving the board revision from the
FPGA and mapping the appropriate IRQ.

Signed-off-by: Alistair Popple <alistair@popple.id.au>
Acked-by: Tony Breeds <tony@bakeyournoodle.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>