ARM: dts: meson: add support for the Meson8m2 SoC

This adds a meson8m2.dtsi which simply inherits meson8.dtsi as both SoCs
share most peripherals.
The known differences are:
- Meson8m2's hardware video decoder additionally supports H.265 decoding
- Meson8m2 has the same Gigabit MAC as Meson8b (instead of the 10/100M
  MAC that Meson8 uses)
- Meson8m2 uses the same watchdog register layout/bits as Meson8b (using
  the Meson8 watchdog compatible leads to an infinite hang when
  rebooting the machine)
- Meson8m2 uses the same SAR ADC register layout/bits as Meson8b.
  However, it uses the temperature sensor calibration formula (and
  registers) Meson8b which differ from Meson8. This however is currently
  not supported by the meson-saradc driver yet.
- the pin controller is mostly compatible with Meson8, Meson8m2 has
  an additional function on eight pins and removes the "VGA" function.
  So there's a total of 10 pins which are slightly changed, which is why
  there's a separate compatible for the pin controller
- a separate compatible for the clock controller is used because at
  least the Mali clock tree (not supported yet) is the same as on GXBB
  while Meson8 and Meson8b have a reduced/older version of the Mali
  clock tree.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>