regulator: core: Allow specific minimal selector for starting linear mapping

Some drivers (at least 3 drivers) have such variant of linear mapping that
the first few selectors are invalid and the reset are linear mapping.
Let's support this case in core.

This patch adds linear_min_sel in struct regulator_desc,
so we can allow specific minimal selector for starting linear mapping.
Then extends regulator_[map|list]_voltage_linear() to support this feature.

Note that for selectors less than min_linear_index, we need count them to
n_voltages so regulator_list_voltage() won't fail while checking the boundary
for selector before calling list_voltage callback.

Signed-off-by: Axel Lin <axel.lin@ingics.com>
Signed-off-by: Mark Brown <broonie@opensource.wolfsonmicro.com>
diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index e872c8b..02a249b 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -1897,6 +1897,10 @@
 {
 	if (selector >= rdev->desc->n_voltages)
 		return -EINVAL;
+	if (selector < rdev->desc->linear_min_sel)
+		return 0;
+
+	selector -= rdev->desc->linear_min_sel;
 
 	return rdev->desc->min_uV + (rdev->desc->uV_step * selector);
 }
@@ -2120,6 +2124,8 @@
 	if (ret < 0)
 		return ret;
 
+	ret += rdev->desc->linear_min_sel;
+
 	/* Map back into a voltage to verify we're still in bounds */
 	voltage = rdev->desc->ops->list_voltage(rdev, ret);
 	if (voltage < min_uV || voltage > max_uV)