arm: mvebu: Add support for the Armada XP theadorable board

arm: mvebu: Add support for the Armada XP theadorable board

This patch adds support for the Armada XP (MV78260) based theadorable
board. Its equipped with onboard DDR3, UART, ethernet, I2C, SPI NOR,
LCD and SATA (SSD) interfaces / devices.

Two defconfigs are added:

theadorable_defconfig:
The production U-Boot version with a stripped down drivers and feature
list. This removes networking, USB and PCI support.

theadorable_debug_defconfig:
The debugging / testing U-Boot version with full support for all drivers.

Signed-off-by: Stefan Roese <sr@denx.de>
Cc: Luka Perkov <luka.perkov@sartura.hr>
Signed-off-by: Stefan Roese <sr@denx.de>