arm: kirkwood: openblocks-a6: add support for Init button

The Kirkwood-based PlatHome OpenBlocks A6 board has an Init button
connected to MPP pin 38. This commit adds support for this button.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Tested-by: Atsushi Yamagata <yamagata@plathome.co.jp>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>