regulator: add a new API regulator_set_voltage_tol()

There are some use cases where a voltage range could be reasonably
specified by a target voltage and tolerance.  Add a new API
regulator_set_voltage_tol() wrapping regulator_set_voltage() call
to ease the users.
Signed-off-by: Shawn Guo <shawn.guo@linaro.org>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>

include/linux/regulator/consumer.h

@@ -352,4 +352,11 @@ static inline void regulator_set_drvdata(struct regulator *regulator,
 
 #endif
 
+static inline int regulator_set_voltage_tol(struct regulator *regulator,
+					    int new_uV, int tol_uV)
+{
+	return regulator_set_voltage(regulator,
+				     new_uV - tol_uV, new_uV + tol_uV);
+}
+
 #endif