dt-bindings: mfd: Add Khadas Microcontroller bindings

This Microcontroller is present on the Khadas VIM1, VIM2, VIM3 and Edge
boards.

It has multiple boot control features like password check, power-on
options, power-off control and system FAN control on recent boards.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>