ARM: dts: meson8m2: Use the Meson8m2 specific USB2 PHY compatible

Use the Meson8m2 specific USB2 PHY compatible string. The 3.10 vendor
kernel has at least one known difference between Meson8 and Meson8m2:
Meson8m2 sets the ACA_ENABLE bit while Meson8 doesn't.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>
Signed-off-by: Kevin Hilman <khilman@baylibre.com>
Link: https://lore.kernel.org/r/20200515202520.1487514-1-martin.blumenstingl@googlemail.com