arm: mvebu: Armada XP MV78230 has two cores, not one

Contrary to our understanding at the time armada-xp-mv78230.dtsi was
written, the MV78230 variant of the Armada XP SoC has two cores and
not one. This patch updates the .dtsi file to take into account this
reality.

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