leds: Provide devm_of_led_get_optional()

Add an optional variant of devm_of_led_get(). It behaves the same as
devm_of_led_get() except where the LED doesn't exist. In this case,
instead of returning -ENOENT, the function returns NULL.

Signed-off-by: Jean-Jacques Hiblot <jjhiblot@traphandler.com>
Reviewed-by: Andy Shevchenko <andy.shevchenko@gmail.com>
Link: https://lore.kernel.org/r/20230728153731.3742339-2-jjhiblot@traphandler.com
Signed-off-by: Lee Jones <lee@kernel.org>