platform/x86: pcengines-apuv2: fix simswap GPIO assignment

The mapping entry has to hold the GPIO line index instead of
controller's register number.

Fixes: 5037d4dd

 ("platform/x86: pcengines-apuv2: wire up simswitch gpio as led")
Signed-off-by: Enrico Weigelt, metux IT consult <info@metux.net>
Signed-off-by: Andy Shevchenko <andriy.shevchenko@linux.intel.com>