dt-bindings: clock: cdce925: Add regulator documentation

The cdce925 has two separate supply pins. Document the bindings
for them.

Signed-off-by: Phil Reid <preid@electromag.com.au>
Link: https://lkml.kernel.org/r/1561691950-42154-2-git-send-email-preid@electromag.com.au


Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Stephen Boyd <sboyd@kernel.org>