gpio: Add support for DM GPIO for Kirkwood

gpio: Add support for DM GPIO for Kirkwood

The Armada driver also works on Nedap's custom Kirkwood board with a
Marvell 88F6180 CPU. The original commit of that driver,
commit 704d9a645e17 ("gpio: Add DM GPIO driver for Marvell MVEBU"),
also mentions that this driver would be suitable for Kirkwood. This
does not completely replace the Kirkwood specific driver as there
are still boards depending on that driver.

Signed-off-by: Harm Berntsen <harm.berntsen@nedap.com>
CC: Stefan Roese <sr@denx.de>,
Reviewed-by: Stefan Roese <sr@denx.de>