platform/x86: pcengines-apuv2: add mpcie reset gpio export

On APUx we have also mpcie2/mpcie3 reset pins. To make it possible to reset
the ports from the userspace, add the definition to this platform
device. The gpio can then be exported by the legancy gpio subsystem to
toggle the mpcie reset pin.

Signed-off-by: Florian Eckert <fe@dev.tdt.de>
Acked-by: Enrico Weigelt <info@metux.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>