drivers: led: make more API callbacks optional

For the LED controller without any hardware blinking capability this
doesn't really make sense to make the blink callback mandatory. It is
better to let it undefined. This way this will allow the LED API to
fall back to a common software blink implementation.

In addition some controllers with RGB LEDs don't have the capability
to configure the LED brightness but only the channel/color levels.
Their drivers may only implement the set_color callback.

That's why this patch makes the blink and set_brightness LED API
callbacks optional.

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