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>