Documentation: arm: add hardware datasheet reference for Marvell Armada XP

Some time ago, the "Function Specifications" for the Marvell Armada XP
processor has been released. However, the "Hardware Specifications"
were not released at the same time. They have now been released
publicly, so this commit updates the Marvell documentation file with
the references to this new documentation.

Three documents are available, one for each of the three variants of
Armada XP: 78230, 78260 and 78460.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Link: https://lkml.kernel.org/r/1410896277-32476-1-git-send-email-thomas.petazzoni@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>