regulator: Add pull down support

Some regulators need to be configured to pull down a resistor
when the regulator is disabled. Add an op (set_pull_down) and a
DT property + constraint to support this.

Signed-off-by: Stephen Boyd <sboyd@codeaurora.org>
Signed-off-by: Mark Brown <broonie@kernel.org>
diff --git a/drivers/regulator/core.c b/drivers/regulator/core.c
index c8d5e2b..60fcfba 100644
--- a/drivers/regulator/core.c
+++ b/drivers/regulator/core.c
@@ -1051,6 +1051,14 @@
 		}
 	}
 
+	if (rdev->constraints->pull_down && ops->set_pull_down) {
+		ret = ops->set_pull_down(rdev);
+		if (ret < 0) {
+			rdev_err(rdev, "failed to set pull down\n");
+			goto out;
+		}
+	}
+
 	print_constraints(rdev);
 	return 0;
 out: