ARM: Kirkwood: Add 6192 DTSI file

This file is mainly a copy of kirkwood-6281.dtsi.
The pinctrl seems to be the same.
These platforms differs only with their CPU, memory capabilities and the
number of GPIO available (36 on 6192, 50 on 6281).

Signed-off-by: Maxime Hadjinlian <maxime.hadjinlian@gmail.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>