ARM: mvebu: Add the core-divider clock to Armada 370/XP

The Armada 370/XP SoC has a clock provider called "Core Divider",
that is derived from a fixed 2 GHz PLL clock.

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>