Convert the binding to DT schema format.

I slightly modified the binding by allowing regulator-[0-9] and cable-[0-9]
instead of regulator@[0-9] and cable@[0-9], so that DT compiler does not
complain about missing 'reg' property. The driver actually ignores the
nodename and can handle both styles.
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Sebastian Reichel <sebastian.reichel@collabora.com>