microblaze/pci: Drop PowerPC-ism from irq parsing

The Microblaze PCI code copied the PowerPC irq handling, but powerpc
needs to handle broken device trees that are not present on Microblaze.
This patch removes the powerpc special case and replaces it with a
direct of_irq_parse_and_map_pci() call.

Signed-off-by: Grant Likely <grant.likely@linaro.org>
Acked-by: Michal Simek <monstr@monstr.eu>