arm: kirkwood: add SoC-level Device Tree data for PCIe interfaces

This commit adds Device Tree details to enable the PCIe interfaces on
Kirkwood. The 6281 has one PCIe interface, the 6282 has two PCIe
interfaces.

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>