drivers: led: add shell support

This patch introduces the "led" shell command. This allows to run the
LED API functions (and to test the LED drivers) from the Zephyr shell.
The following subcommands are supported:

- on
- off
- get_info
- set_brightness
- set_color
- set_channel
- write_channels

Signed-off-by: Simon Guinot <simon.guinot@seagate.com>