ARM: iop32x/n2100: fix PCI IRQ mapping

Booting 4.20 on a TheCUS N2100 results in a kernel oops while probing
PCI, due to n2100_pci_map_irq() having been discarded during boot.

Signed-off-by: Russell King <rmk+kernel@armlinux.org.uk>
Cc: stable@vger.kernel.org # 2.6.18+
Signed-off-by: Arnd Bergmann <arnd@arndb.de>