clk: mvebu: Add Core Divider clock

This commit introduces a new group of clocks present in Armada 370/XP
SoCs (called "Core Divider" clocks) and add a provider for them.
The only clock supported for now is the NAND clock (ndclk), but the
infrastructure to add the rest is already set.

Reviewed-by: Gregory CLEMENT <gregory.clement@free-electrons.com>
Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Signed-off-by: Mike Turquette <mturquette@linaro.org>