leds: gpio: Support the "panic-indicator" firmware property

Calling a GPIO LEDs is quite likely to work even if the kernel
has paniced, so they are ideal to blink in this situation.
This commit adds support for the new "panic-indicator"
firmware property, allowing to mark a given LED to blink on
a kernel panic.

Signed-off-by: Ezequiel Garcia <[e-mail]>
Reviewed-by: Matthias Brugger <[e-mail]>
Acked-by: Pavel Machek <[e-mail]>
Signed-off-by: Jacek Anaszewski <[e-mail]>

Documentation/devicetree/bindings/leds/leds-gpio.txt

···
   property is not present.
 - retain-state-suspended: (optional) The suspend state can be retained.Such
   as charge-led gpio.
+- panic-indicator : (optional)
+  see Documentation/devicetree/bindings/leds/common.txt
 
 Examples:
 

drivers/leds/leds-gpio.c

···
 	led_dat->cdev.brightness = state ? LED_FULL : LED_OFF;
 	if (!template->retain_state_suspended)
 		led_dat->cdev.flags |= LED_CORE_SUSPENDRESUME;
+	if (template->panic_indicator)
+		led_dat->cdev.flags |= LED_PANIC_INDICATOR;
 
 	ret = gpiod_direction_output(led_dat->gpiod, state);
 	if (ret < 0)
···
 
 		if (fwnode_property_present(child, "retain-state-suspended"))
 			led.retain_state_suspended = 1;
+		if (fwnode_property_present(child, "panic-indicator"))
+			led.panic_indicator = 1;
 
 		ret = create_gpio_led(&led, &priv->leds[priv->num_leds],
 				      dev, NULL);

include/linux/leds.h

···
 	unsigned 	gpio;
 	unsigned	active_low : 1;
 	unsigned	retain_state_suspended : 1;
+	unsigned	panic_indicator : 1;
 	unsigned	default_state : 2;
 	/* default_state should be one of LEDS_GPIO_DEFSTATE_(ON|OFF|KEEP) */
 	struct gpio_desc *gpiod;