[ARM] Orion: fix ->map_irq() PCIe bus number check

The current orion5x board ->map_irq() routines check whether a
given bus number lives on the PCIe controller by comparing it with
the PCIe controller's primary bus number.  This doesn't work in
case there are multiple buses in the PCIe domain, i.e. if there
exists a PCIe bridge on the primary PCIe bus.

This patch adds a helper function (orion5x_pci_map_irq()) that
returns the IRQ number for the given PCI device if that device has
a hard-wired IRQ, or -1 otherwise, and makes each board's
->map_irq() function use this helper function.

Signed-off-by: Lennert Buytenhek <buytenh@marvell.com>
Signed-off-by: Nicolas Pitre <nico@marvell.com>