leds: Modify PC Engines apu/apu2 driver to support apu3

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: Raffaello D. Di Napoli <rafdev@dinapo.li>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>