arm: mvebu: Add SPI flash on Armada XP-GP board

This patch adds an SPI master device node for Armada XP-GP board.
This master node is an SPI flash controller 'n25q128a13'.

Since there is no 'partitions' node declared, one full sized
partition named as the device will be created.

Cc: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Cc: Lior Amsalem <alior@marvell.com>
Tested-by: Gregory Clement <gregory.clement@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Acked-by: Gregory Clement <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>