dt-bindings: clock: at91: new sckc bindings

Remove the need for child nodes in the sckc binding to be able to remove
dtc warnings and have a more modern binding.

Also document optional properties.

Cc: Rob Herring <robh@kernel.org>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Alexandre Belloni <alexandre.belloni@bootlin.com>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>