leds: leds-menf21bmc: Introduce MEN 14F021P00 BMC LED driver

Added driver to support the 14F021P00 BMC LEDs.
The BMC is a Board Management Controller including four LEDs which
can be switched on and off.

Signed-off-by: Andreas Werner <andreas.werner@men.de>
Acked-by: Bryan Wu <cooloney@gmail.com>
Signed-off-by: Lee Jones <lee.jones@linaro.org>