mfd: menf21bmc: Introduce MEN 14F021P00 BMC MFD Core driver

The MEN 14F021P00 Board Management Controller provides an
I2C interface to the host to access the feature implemented in the BMC.
The BMC is a PIC Microntroller assembled on CPCI Card from MEN Mikroelektronik
and on a few Box/Display Computer.

Added MFD Core driver, supporting the I2C communication to the device.

The MFD driver currently supports the following features:
 	- Watchdog
 	- LEDs
	- Hwmon (voltage monitoring)

Signed-off-by: Andreas Werner <andreas.werner@men.de>
Signed-off-by: Lee Jones <lee.jones@linaro.org>