ARM: mvebu: fix gated clock documentation

The gated clock documentation referred only to the Orion SoC whereas
it also applied for the Armada 370/XP SoC. This commit updates the
introduction text and also the list of the compatible strings.

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