drivers: led: introduce led_get_info syscall

When supported by the driver the led_get_info syscall allows a user
to retrieve the following information about each LED available:

- The LED label.
- The number of colors/channels.
- And for a multicolor LED a pointer to a channel-color mapping.

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>
Signed-off-by: Maxime Bittan <maxime.bittan@seagate.com>