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

This patch add support for the SPI flash M25P64 which is present on
the Armada XP DB board. This flash stores the bootloader and its
environment.

Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>