ARM: Kirkwood: Add DTSI files for pinctrl

There are a couple of different variants of Kirkwood, which differ in
the pin muxing. These DTSI files set the correct compatibility and
define commonly used groups of pins, which board dbs files can
reference.

Signed-off-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>