ARM: mvebu: prepare mvebu_pm_store_bootinfo() to support multiple SoCs

As we are going to introduce support for Armada 38x in pm.c, split out
the Armada XP part of mvebu_pm_store_bootinfo() into
mvebu_pm_store_armadaxp_bootinfo(), and make the former retunr an
error when an unsupported SoC is used.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>