dt-bindings: mfd: qcom,spmi-pmic: Fix regulator node schema

The regulators node of Qualcomm SPMI PMIC represents sub-device, so it
has its own compatible, multiple regulators and uses dedicated bindings.

Fixes: 3f5117be ("dt-bindings: mfd: convert to yaml Qualcomm SPMI PMIC")
Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@linaro.org>
Acked-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20220828084341.112146-14-krzysztof.kozlowski@linaro.org