ARM: kirkwood: Separate board-dt from common and pcie code.

In order to be able to move DT support into mach-mvebu, the DT code
needs to be cleanly separated from common and pcie code. Import the
needed bits of these files into board-dt.c. The "common" code then
becomes purely legacy, supporting non-DT boards, so reflect this in
the Makefile targets.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Jason Gunthorpe <jgunthorpe@obsidianresearch.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>