regulator: add missing prototype for regulator_is_supported_voltage

avoids needs for CONFIG_REGULATOR in sdhci.c

Signed-off-by: Philip Rakity <prakity@nvidia.com>
Reviewed-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Eric Miao <eric.y.miao@gmail.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/regulator/consumer.h

@@ -358,6 +358,10 @@ static inline void regulator_set_drvdata(struct regulator *regulator,
 {
 }
 
+static inline int regulator_count_voltages(struct regulator *regulator)
+{
+	return 0;
+}
 #endif
 
 static inline int regulator_set_voltage_tol(struct regulator *regulator,