regulator: bindings: Add properties for coupled regulators

Some regulators require keeping their voltage spread below defined
max_spread.

Add properties to provide information on regulators' coupling.

Signed-off-by: Maciej Purski <m.purski@samsung.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Mark Brown <broonie@kernel.org>