PCI: designware: Use new OF interrupt mapping when possible

Use new OF interrupt mapping (of_irq_parse_and_map_pci()) when possible.
This is the recommended method of doing the IRQ mapping.  For old
devicetrees we fall back to the previous practice.

This makes INTB, INTC, and INTD work on i.MX.

Tested-by: Tim Harvey <tharvey@gateworks.com>
Signed-off-by: Lucas Stach <l.stach@pengutronix.de>
Signed-off-by: Bjorn Helgaas <bhelgaas@google.com>
Reviewed-by: Marek Vasut <marex@denx.de>
Acked-by: Arnd Bergmann <arnd@arndb.de>
Acked-by: Jingoo Han <jg1.han@samsung.com>