arm: kirkwood: convert db-88f6281/db-88f6282 to the Device Tree

This commit converts the Marvell DB-88F6281/DB-88F6282 board to the
Device Tree. In fact, the code was supporting two different boards:
one with the 6281 SoC variant, and one with the 6282 SoC variant. The
difference between the two being that the 6281 has one PCIe interface,
and the 6282 has two PCIe interfaces.

In order to handle that with the Device Tree, we create a
'kirkwood-db.dtsi' file that contains the definitions common to both
boards, and 'kirkwood-db-88f6281.dts' and 'kirkwood-db-88f6282.dts'
for the definitions specific to each board. This is similar to what is
done for the QNAP TS219 Kirkwood platform.

We have kept one single Kconfig option, just like it was before.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>