regulator: max8973: initial DT support

This patch adds primitive DT support to the max8973 regulator driver. None
of the configuration parameters, supported in the platform data are yet
available in DT, therefore no configuration is performed if booting with
no platform data. This means, that DT instantiation can only be used on
boards, where no run-time configuration of the chip is required. In such
cases the driver can be used to scale its output voltage. In the future
support for configuration parameters should be added.

Signed-off-by: Guennadi Liakhovetski <g.liakhovetski+renesas@gmail.com>
Signed-off-by: Mark Brown <broonie@linaro.org>