ARM: mvebu: Add button on Armada 370 Reference Design board

The Marvell Armada 370 Reference Design board has a software-controlled
button on the front side, marked as "SW".
This patch adds minimal support for this button.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Florian Fainelli <florian@openwrt.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>