pci: mvebu: enable driver usage on Kirkwood

We allow the pci-mvebu driver to be compiled on the Kirkwood platform,
and add the 'marvell,kirkwood-pcie' as a compatible string supported
by the driver.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>