ARM: mvebu: move ARMADA_XP_MAX_CPUS to armada-370-xp.h

The ARMADA_XP_MAX_CPUS definition was in common.h, which as its name
says, is common to all mvebu SoCs. It is more logical to have this XP
specific definition in the already existing armada-370-xp.h header
file, especially in preparation to the addition of the support for
other SOCs in mach-mvebu.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Acked-by: Andrew Lunn <andrew@lunn.ch>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>