dt-bindings: arm: amlogic: add support for the Meson8m2 SoC

The Meson8m2 SoC is an updated version of the Meson8 SoC. Some of the
peripherals are updated to be register-compatible with Meson8b.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Reviewed-by: Rob Herring <robh@kernel.org>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>