leds: led-class: Add __devm_led_get() helper

Add a __devm_led_get() helper which registers a passed in led_classdev
with devm for unregistration.

This is a preparation patch for adding a generic (non devicetree specific)
devm_led_get() function.

Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Reviewed-by: Linus Walleij <linus.walleij@linaro.org>
Signed-off-by: Hans de Goede <hdegoede@redhat.com>
Signed-off-by: Lee Jones <lee@kernel.org>
Link: https://lore.kernel.org/r/20230120114524.408368-4-hdegoede@redhat.com