arm: mvebu: split Kconfig options for Armada 370 and XP

Until now, all the code for Armada 370 and XP was common, so we had a
single Kconfig option to support all boards using both SoCs. With the
addition of pinctrl drivers, this situation has changed: those two
SoCs are radically different in terms of pinctrl, so they have two
separate drivers. Since pinctrl drivers are typically select-ed from
the SoC Kconfig option, it makes sense to split the 370/XP option into
two separate options: one for Armada 370 and another for Armada XP.

We keep an hidden option selected by both ARMADA_370 and ARMADA_XP in
order to easily compile common code.

A followup patch actually makes use of this split to select the
appropriate pinctrl drivers.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Acked-by: Gregory Clement <gregory.clement@free-electrons.com>
Acked-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Tested-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>
Reviewed-by: Stephen Warren <swarren@wwwdotorg.org>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>