regulator: Change regulator-coupled-max-spread property

Redefine binding for regulator-coupled-max-spread property in a way that
max-spread values are defined per regulator couple instead of defining
single max-spread for the whole group of coupled regulators.

With that change the following regulators coupling configuration will be
possible:

regA: regulatorA {
	regulator-coupled-with = <&regB &regC>;
	regulator-coupled-max-spread = <100000 300000>;
};

regB: regulatorB {
	regulator-coupled-with = <&regA &regC>;
	regulator-coupled-max-spread = <100000 200000>;
};

regC: regulatorC {
	regulator-coupled-with = <&regA &regB>;
	regulator-coupled-max-spread = <300000 200000>;
};

Signed-off-by: Dmitry Osipenko <digetx@gmail.com>
Signed-off-by: Mark Brown <broonie@kernel.org>