dt-bindings: phy: Add the Amlogic Meson8 HDMI TX PHY bindings

Amlogic Meson8, Meson8b and Meson8m2 all include an identical (or at
least very similar) HDMI TX PHY. The PHY registers are part of the HHI
register area.

Signed-off-by: Martin Blumenstingl <martin.blumenstingl@googlemail.com>