dt-bindings: mfd: Add binding for Netronix embedded controller

This EC is found in e-book readers of multiple brands (e.g. Kobo,
Tolino), and is typically implemented as a TI MSP430 microcontroller.

It controls different functions of the system, such as power on/off,
RTC, PWM for the backlight. The exact functionality provided can vary
between boards.

Signed-off-by: Jonathan Neuschäfer <j.neuschaefer@gmx.net>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>