MAINTAINERS: Adds Andreas Werner to maintainers list for MEN F21BMC

Added maintainer for the following MEN F21BMC drivers:
	- menf21bmc (MFD)
	- menf21bmc_wdt (Watchdog)
	- menf21bmc_hwmon (HWMON)
	- leds-menf21bmc (LED)

Signed-off-by: Andreas Werner <andreas.werner@men.de>
Acked-by: Guenter Roeck <linux@roeck-us.net>
Acked-by: Bryan Wu <cooloney@gmail.com>
Acked-by: Wim Van Sebroeck <wim@iguana.be>
Signed-off-by: Lee Jones <lee.jones@linaro.org>