dt: Add additional option bindings for IDT VersaClock

The VersaClock driver now supports some additional bindings to support
child nodes which can configure optional settings like mode, voltage
and slew.

This patch updates the binding document to describe what is available
in the driver.

Signed-off-by: Adam Ford <aford173@gmail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Link: https://lore.kernel.org/r/20200603154329.31579-2-aford173@gmail.com


Signed-off-by: Stephen Boyd <sboyd@kernel.org>