regulator: gpio: Honor regulator-boot-on property

When requesting the enable GPIO, the driver should do so with the
correct output level matching some expected state. This is especially
important if the regulator is a critical one, such as a supply for
the boot CPU. This is currently done by checking for the enable-at-boot
property, but this is not documented in the device tree binding, nor
does it match the common regulator properties.

Honor the common regulator-boot-on property by checking the boot_on
constraint setting within the DT probe path. This is the same as what
is done in the fixed regulator driver.

Also add a comment stating that the enable-at-boot property should not
be used.

Fixes: 006694d ("regulator: gpio-regulator: Allow use of GPIO controlled regulators though DT")
Signed-off-by: Chen-Yu Tsai <wens@csie.org>
Link: https://lore.kernel.org/r/20200720132809.26908-1-wens@kernel.org
Signed-off-by: Mark Brown <broonie@kernel.org>

drivers/regulator/gpio-regulator.c

@@ -148,6 +148,13 @@ of_get_gpio_regulator_config(struct device *dev, struct device_node *np,
 
 	config->supply_name = config->init_data->constraints.name;
 
+	if (config->init_data->constraints.boot_on)
+		config->enabled_at_boot = true;
+
+	/*
+	 * Do not use: undocumented device tree property.
+	 * This is kept around solely for device tree ABI stability.
+	 */
 	if (of_property_read_bool(np, "enable-at-boot"))
 		config->enabled_at_boot = true;