leds: lp8860: Various fixes to align with LED framework

Update the driver to conform with the LED framework:
 - use devm_led_classdev_register
 - destroy mutex on exit
 - remove dependency on CONFIG_OF in the driver and move
   to the Kconfig
 - update the MODULE_LICENSE to GPL v2
 - remove setting of MAX brightness as the LED framework
   does this.

Signed-off-by: Dan Murphy <dmurphy@ti.com>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>