led: pwm: Use NOP uclass driver for top-level node

The top level DT node of pwm-leds is not a LED itself, bind NOP uclass
driver to it, and bind different LED uclass driver to its subnodes which
represent the actual LEDs. This change removes the top-level node from
the 'led list' command output and is based on the commit 01074697
("led: gpio: Use NOP uclass driver for top-level node").

Signed-off-by: Stefan Herbrechtsmeier <stefan.herbrechtsmeier@weidmueller.com>