regulator: core: Add regulator_map_voltage_ascend() API

A lot of regulator hardware has ascendant voltage list.
This patch adds regulator_map_voltage_ascend() and export it.

Drivers that have ascendant voltage list can use this as their map_voltage()
operation, this is more efficient than default regulator_map_voltage_iterate()
function.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/include/linux/regulator/driver.h b/include/linux/regulator/driver.h
index 7df93f5..2acdc66 100644
--- a/include/linux/regulator/driver.h
+++ b/include/linux/regulator/driver.h
@@ -329,6 +329,8 @@
 				  int min_uV, int max_uV);
 int regulator_map_voltage_iterate(struct regulator_dev *rdev,
 				  int min_uV, int max_uV);
+int regulator_map_voltage_ascend(struct regulator_dev *rdev,
+				  int min_uV, int max_uV);
 int regulator_get_voltage_sel_regmap(struct regulator_dev *rdev);
 int regulator_set_voltage_sel_regmap(struct regulator_dev *rdev, unsigned sel);
 int regulator_is_enabled_regmap(struct regulator_dev *rdev);