arm: mvebu: Add SMP support for Armada XP

This enables SMP support on the Armada XP processor. It adds the
mandatory functions to support SMP such as: the SMP initialization
functions in platsmp.c, the secondary CPU entry point in headsmp.S and
the CPU hotplug initial support in hotplug.c.

Signed-off-by: Yehuda Yitschak <yehuday@marvell.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Reviewed-by: Will Deacon <will.deacon@arm.com>