ARM: mvebu: Add Core Divider clock device-tree binding

The Armada 370/XP SoCs have a Core Divider clock providing
several clocks. For now, only the NAND clock is supported.

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>