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/of_regulator.c b/drivers/regulator/of_regulator.c
index 482a86f..c3433db 100644
--- a/drivers/regulator/of_regulator.c
+++ b/drivers/regulator/of_regulator.c
@@ -67,6 +67,8 @@
 	if (!constraints->always_on) /* status change should be possible. */
 		constraints->valid_ops_mask |= REGULATOR_CHANGE_STATUS;
 
+	constraints->pull_down = of_property_read_bool(np, "regulator-pull-down");
+
 	if (of_property_read_bool(np, "regulator-allow-bypass"))
 		constraints->valid_ops_mask |= REGULATOR_CHANGE_BYPASS;