regulator: adapt fixed regulator driver to dt

The fixed regulator driver uses of_get_fixed_voltage_config()
to extract fixed_voltage_config structure contents from device tree.

Also add documenation for additional bindings for fixed
regulators that can be passed through dt.

Signed-off-by: Rajendra Nayak <rnayak@ti.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>