regulator: dt-bindings: maxim,max8973: convert to dtschema

Convert the Maxim MAX8973 regulator to DT schema format.  Extend the
examples with more advanced one for MAX77621 copied from kernel's
nvidia/tegra210-smaug.dts, licensed under GPL-2.0.

Signed-off-by: Krzysztof Kozlowski <krzysztof.kozlowski@canonical.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Message-Id: <20211008114755.148279-1-krzysztof.kozlowski@canonical.com>
Signed-off-by: Mark Brown <broonie@kernel.org>