platform/x86: pcengines-apuv2: wire up simswitch gpio as led

The APU3+ boards have two SIM sockets, while only one of them
can be routed to the mpcie slots at a time. Selection is done
via simswap gpio.

We currently don't have a fitting subsystem for those cases yet,
so just wire it up to a LED for the time being. While this isn't
really semantically correct, it's a good compromise.

Signed-off-by: Enrico Weigelt <info@metux.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>