leds-lp55xx: support device specific attributes

To support device specific attributes, new common driver function is added.
 Eventually those are created on registering the sysfs with common dev attrs.
 Furthermore, this patch makes adding device attributes simple in each driver.

Signed-off-by: Milo(Woogyom) Kim <milo.kim@ti.com>
Signed-off-by: Bryan Wu <cooloney@gmail.com>

drivers/leds/leds-lp55xx-common.h

@@ -45,6 +45,7 @@ struct lp55xx_reg {
  * @set_led_current    : LED current set function
  * @firmware_cb        : Call function when the firmware is loaded
  * @run_engine         : Run internal engine for pattern
+ * @dev_attr_group     : Device specific attributes
  */
 struct lp55xx_device_config {
 	const struct lp55xx_reg reset;
@@ -65,6 +66,9 @@ struct lp55xx_device_config {
 
 	/* used for running firmware LED patterns */
 	void (*run_engine) (struct lp55xx_chip *chip, bool start);
+
+	/* additional device specific attributes */
+	const struct attribute_group *dev_attr_group;
 };
 
 /*