arm64: dts: meson-khadas-vim3: add Khadas MCU nodes

Add the Khadas MCU node with active FAN thermal nodes for all the
Khadas VIM3 variants.

Signed-off-by: Neil Armstrong <narmstrong@baylibre.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Reviewed-by: Amit Kucheria <amit.kucheria@linaro.org>
Link: https://lore.kernel.org/r/20200713065931.19845-1-narmstrong@baylibre.com