drivers: regulator: fixed: add u-boot, off-on-delay-us

Add u-boot,off-on-delay-us for fixed regulator.

Depends on board design, the gpio regulator sometimes
connects with a big capacitance. When need to off, then
on the regulator, if there is no enough delay,
the voltage does not drop to 0, so introduce this
property to handle such case.

Signed-off-by: Peng Fan <peng.fan@nxp.com>
Reviewed-by: Simon Glass <sjg@chromium.org>
Cc: Masahiro Yamada <yamada.masahiro@socionext.com>

doc/device-tree-bindings/regulator/fixed.txt

@@ -11,6 +11,7 @@ Required properties:
 Optional properties:
 - gpio: GPIO to use for enable control
 - startup-delay-us: startup time in microseconds
+- u-boot,off-on-delay-us: off delay time in microseconds
 - regulator constraints (binding info: regulator.txt)
 - enable-active-high: Polarity of GPIO is Active high. If this property
   is missing, the default assumed is Active low.

drivers/power/regulator/fixed.c

@@ -16,6 +16,7 @@
 struct fixed_regulator_platdata {
 	struct gpio_desc gpio; /* GPIO for regulator enable control */
 	unsigned int startup_delay_us;
+	unsigned int off_on_delay_us;
 };
 
 static int fixed_regulator_ofdata_to_platdata(struct udevice *dev)
@@ -50,6 +51,8 @@ static int fixed_regulator_ofdata_to_platdata(struct udevice *dev)
 	/* Get optional ramp up delay */
 	dev_pdata->startup_delay_us = dev_read_u32_default(dev,
 							"startup-delay-us", 0);
+	dev_pdata->off_on_delay_us =
+			dev_read_u32_default(dev, "u-boot,off-on-delay-us", 0);
 
 	return 0;
 }
@@ -123,6 +126,9 @@ static int fixed_regulator_set_enable(struct udevice *dev, bool enable)
 		udelay(dev_pdata->startup_delay_us);
 	debug("%s: done\n", __func__);
 
+	if (!enable && dev_pdata->off_on_delay_us)
+		udelay(dev_pdata->off_on_delay_us);
+
 	return 0;
 }