ARM: dts: am335x-sl50: update backlight nodes

This patch updates the backlight nodes to improve the support and describe
better how hardware is done. The changes done were:

 * Use PWM_POLARITY_INVERTED instead of the hardcoded number.
 * Add pinctrl configuration.
 * Add the enable gpio definition.
 * Add the power supply definition.
 * Add more brightness levels.

Signed-off-by: Enric Balletbo i Serra <enric.balletbo@collabora.com>
Signed-off-by: Tony Lindgren <tony@atomide.com>