dt-bindings: marvell: Add documentation for the Armada 3700 AVS binding

Extend the documentation of the Armada 37xx SoC with the Adaptive Voltage
Scaling (AVS) registers.

Signed-off-by: Gregory CLEMENT <gregory.clement@bootlin.com>
Acked-by: Viresh Kumar <viresh.kumar@linaro.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Rafael J. Wysocki <rafael.j.wysocki@intel.com>