leds: Add more product/board names for PC Engines APU2

PC Engines "legacy" coreboot 4.0.8 changed the product/board name
from "APU2" to "apu2".
PC Engines "mainline" coreboot uses, instead, "PC Engines apu2" as
product/board name.

This commit adds the 2 variants ("apu2" and "PC Engines apu2") of
product/board name to be compatible with all the APU2 BIOSes.

Fixes: 3faee942 ("leds: Add driver for PC Engines APU/APU2 LEDs")
Signed-off-by: Timothy Redaelli <tredaelli@redhat.com>
Acked-by: Pavel Machek <pavel@ucw.cz>
Signed-off-by: Jacek Anaszewski <jacek.anaszewski@gmail.com>