Add a generic [devm_]led_get() method which can be used on both devicetree
and non devicetree platforms to get a LED classdev associated with
a specific function on a specific device, e.g. the privacy LED associated
with a specific camera sensor.

Note unlike of_led_get() this takes a string describing the function
rather then an index. This is done because e.g. camera sensors might
have a privacy LED, or a flash LED, or both and using an index
approach leaves it unclear what the function of index 0 is if there is
only 1 LED.

This uses a lookup-table mechanism for non devicetree platforms.
This allows the platform code to map specific LED class_dev-s to a specific
device,function combinations this way.

For devicetree platforms getting the LED by function-name could be made
to work using the standard devicetree pattern of adding a -names string
array to map names to the indexes.
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/20230120114524.408368-5-hdegoede@redhat.com