ARM: mvebu: Add a 2 GHz fixed-clock Armada 370/XP

Armada 370/XP SoCs have a 2 GHz fixed PLL that is used to feed
other clocks. This commit adds a DT representation of this clock
through a fixed-clock compatible node.

Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Jason Cooper <jason@lakedaemon.net>