thermal: Add support for the MCU controlled FAN on Khadas boards

The new Khadas VIM2 and VIM3 boards controls the cooling fan via the
on-board microcontroller.

This implements the FAN control as thermal devices and as cell of the Khadas
MCU MFD driver.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Acked-by: Daniel Lezcano <daniel.lezcano@linaro.org>
Signed-off-by: Lee Jones <lee.jones@linaro.org>