regulator: core: provide fixed voltage in desc for single voltage rail

If given rail has the single voltage (n_voltages = 1) then provide the
rail voltage through regulator descriptor so that core can use this
value for finding voltage.

This will avoid the implementation of the callback for get_voltage() or
list_voltage() callback on regulator driver.

Signed-off-by: Laxman Dewangan <ldewangan@nvidia.com>
Signed-off-by: Mark Brown <broonie@linaro.org>

include/linux/regulator/driver.h

@@ -207,6 +207,7 @@ enum regulator_type {
  * @min_uV: Voltage given by the lowest selector (if linear mapping)
  * @uV_step: Voltage increase with each selector (if linear mapping)
  * @linear_min_sel: Minimal selector for starting linear mapping
+ * @fixed_uV: Fixed voltage of rails.
  * @ramp_delay: Time to settle down after voltage change (unit: uV/us)
  * @volt_table: Voltage mapping table (if table based mapping)
  *
@@ -239,6 +240,7 @@ struct regulator_desc {
 	unsigned int min_uV;
 	unsigned int uV_step;
 	unsigned int linear_min_sel;
+	int fixed_uV;
 	unsigned int ramp_delay;
 
 	const struct regulator_linear_range *linear_ranges;