dt-bindings: add Marvell core PLL and clock divider PMU documentation

Add documentation for the Marvell clock divider driver, which is used
to source clocks for the AXI bus, video decoder, GPU and LCD blocks.

Acked-by: Andrew Lunn <andrew@lunn.ch>
Acked-by: Sebastian Hesselbarth <sebastian.hesselbarth@gmail.com>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Russell King <rmk+kernel@arm.linux.org.uk>
Signed-off-by: Gregory CLEMENT <gregory.clement@free-electrons.com>