PC Engines apu3 is an improved version of the apu2, using the same SoC
and almost everything else.

This patch reuses as much as possible from the apu2 definitions, to
avoid redundancy.
Signed-off-by: NRaffaello D. Di Napoli <rafdev@dinapo.li>
Acked-by: NPavel Machek <pavel@ucw.cz>
Signed-off-by: NJacek Anaszewski <jacek.anaszewski@gmail.com>