regulator: tps65090: Set voltage for fixed regulators

According to the tps65090 data manual [0], the DCDC1 and DCDC2
step-down converters and the LDO's have a fixed output voltage.

Add this information to the driver since these fixed regulators
can be used as parent input supply for switches that don't have
an output voltage defined. So the regulator core needs to fetch
the FET parent output voltage if the child voltage is queried.

[0]: http://www.ti.com/lit/gpn/tps65090



Signed-off-by: Javier Martinez Canillas <javier.martinez@collabora.co.uk>
Signed-off-by: Mark Brown <broonie@linaro.org>