ARM: meson: add support for the Meson8m2 SoCs

Add the newly introduced compatible string for the Meson8m2 SoC.
The existing Meson8 Kconfig entry is updated to also provide support for
the Meson8m2 SoCs. The Meson8m2 SoC is mostly identical to the Meson8 SoC
with just a few peripherals being updated. Both SoCs currently
share the same pinctrl, clock and GPIO IRQ bindings. Thus the existing
Kconfig entry is re-used to avoid duplication (the only cost is building
a few extra DTBs).

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