arm: armada-xp: Add basic support for Marvell Armada XP SoC

This basic support for the Marvell Armada XP is base on the existing kirkwood
support. Which has been generatized by moving some common files into
common marvell locations.

This is in preparation for the upcoming Armada XP MV78460 support.

Signed-off-by: Stefan Roese <sr@denx.de>
Tested-by: Luka Perkov <luka@openwrt.org>