clk: mvebu: Update binding documentation for the core divider clock

The Core Divider clock support two new compatible strings for Armada
375 and Armada 380 SoCs. Add the compatible strings to the documentation.

Signed-off-by: Ezequiel Garcia <ezequiel.garcia@free-electrons.com>
Link: https://lkml.kernel.org/r/1394742273-5113-7-git-send-email-ezequiel.garcia@free-electrons.com
Signed-off-by: Jason Cooper <jason@lakedaemon.net>